AKA “How I ID California Mammals” 🙂  
Venkat Sankar 

Welcome to the ID guide! This document is especially meant to aid mammal identification from high-quality photos showing key features of the mammal group in question (e.g. face, ears, sides, feet, and tail in rodents; or face, ears, legs, and dorsal fur in bats). Study up on the key features to photograph or watch for in target species before you go so you have the best chance of a solid ID! However, even with high-quality photos, confident species-level ID may still be impossible depending on the group involved & location (for example, some chipmunks or myotis species). This guide is not meant for in-hand ID, for which there are many resources already, but for ID of free-living mammals seen in CA.

Here, we describe key field marks (mostly visual) and habitat preferences for the species listed below, as well as in some cases, sign that may be relevant for ID or sleuthing out a species’ presence in an area. I’ve only covered mammals for which I have a decent level of familiarity within or outside of CA – focusing largely on diverse genera that can pose the greatest ID challenges. For range maps, published field guides are a good option – I prefer the Peterson Guide to Mammals of North America by Fiona Reid. Many of the ID tips I use are based on this guide. Overall, we will not include range info unless it is a useful ID mark, but we do include microhabitats where helpful for finding or IDing species. If you want finer-grained info (sometimes useful for excluding options if you’re at a well-surveyed site), dot-locality maps within CA can be found by trawling databases like MVZ (Arctos), GBIF, and in cases where community ID is fairly trustworthy, iNaturalist (the GBIF layer is awesome, as it combines more reliable specimen records from museum databases with the more recent observers’ pins). In terms of local mammal literature, another exceptional resource is the San Diego Mammal Atlas by Scott Tremor et al. A lot of the records featured there haven’t seem to have made it to the larger databases, and it is an exceptionally useful, truly unique book that enables you to dig deep into the mammals of CA’s most biodiverse county.

Please note that this is a live document and subject to change.

Table of Contents

ID Guide for California Medium and Small-sized Mammals

Lagomorphs (Rabbits & Hares)

Chipmunks

Kangaroo Rats

Pocket Mice

Woodrats

Deer Mice (Peromyscus)

Bats

Lagomorphs (Rabbits & Hares)

1. Brush Rabbit (Sylvilagus bachmani): small cottontail, usually with dark brown or grayish brown fur (I saw one individual with strange dark gray fur in redwood forest in Humboldt Co. once, however) and relatively short ears that lack blackish rims. The fairly small tail is grayish white and inconspicuous. Most abundant in dense cover in the coast ranges, usually coastal scrub or chaparral; less common in similar habitats further inland.

This Brush Rabbit was seen at a site in coastal San Mateo Co. where Desert Cottontails do not occur, but it’s not a great pic to show the ID features. I’ll replace it later, but for now note the lack of a black rim at the top of the ear which differentiates it from (often sympatric) S. audubonii in coastal CA.

2. Pygmy Rabbit (S. idahoensis): very small rabbit of tall, dense big sage (Artemisia tridentata) scrub; today only found in CA in Mono County, where it remains widespread in appropriate habitat. Gray to grayish brown in color, with a compact body. Medium-length ears are thickly covered with long cream-colored fur on the inside. The small, grayish brown tail is inconspicuous, barely contrasting with the body color and usually not seen unless the animal flees. Superficially similar to juvenile cottontails, but with a narrower face and relatively longer ears. Presence can often be determined by searching for its characteristic burrows (the only burrowing rabbit in CA) and tiny fecal pellets under the sage. Colonies are usually found in swales and other areas with deeper sand. Most active early mornings, and readily seen by walking in or staking out such sites.

Note the thick coating of white fur on the inside of the ears of Pygmy Rabbit, as well as the dark grayish brown fur and inconspicuous tail.

3. Mountain Cottontail (S. nuttallii): medium-sized cottontail with fully white tail and long ears with a coating of short fur on the inside. Externally similar to (but slightly smaller) S. audubonii; best distinguished by furry ears. Common in E Sierra, Great Basin, & NE CA.

Top: the short fur coating the inside of the ear distinguishes the Mountain Cottontail from other sympatric Sylvilagus; Bottom: juvenile Mountain Cottontails can be superficially similar to Pygmy Rabbit, but can be identified by the thinner coating of fur in the ears, white tail, and different facial profile & proportionally larger eyes

4. Desert Cottontail (S. audubonii): large, pale cottontail with long ears with a blackish upper rim, grayish brown body fur, and a contrasting orange neck patch. Puffy, large white tail is very distinctive. Widespread primarily in open habitats such as deserts, oak woodland, and grassland in much of CA; locally enters chaparral & coastal scrub (especially in S CA), but S. bachmani is typically more common in dense habitats. This iNaturalist thread provides further details for separating them (https://www.inaturalist.org/posts/39654-brush-rabbit-vs-desert-cottontail).

I need to replace this pretty terrible photo soon (surprising I don’t have a better one of Desert Cottontails, as I see SO MANY) but it does show the orangish nape, pale coloration, near-naked ears, and blackish rims at the top of the ears that characterize the species.

5. Snowshoe Hare (Lepus americanus): small hare, especially the forms we have in CA which almost overlap in size with large Sylvilagus. Identified by relatively short ears with black tips and thin white borders, woolly dark grayish brown dorsal fur and large white feet. All (?) CA populations turn white in winter. Tail inconspicuous, whitish gray in color and usually only seen when the animal flees. Scarce and declining due to climate change & fire damage in montane forests (mostly associated with the Red Fir community) of the C & N Sierra, Cascades, and Klamath Mountains, where it occurs along the margins of wet meadows in willow & alder thickets.

This juvenile Snowshoe Hare is different structurally vs. an adult, but does show the thick woolly grizzled brownish fur, (barely) the white feet, and black-tipped ears with white rims that characterize the species. Also note the white forehead spot, which seems to be a common feature of Snowshoe Hare leverets.

6. Black-tailed Jackrabbit (L. californicus): large, lanky grayish brown hare with very long black-tipped ears and a blackish top of the tail. Widespread in CA, but usually absent from forests.
7. White-tailed Jackrabbit (L. callotis): huge, very robust grayish hare of high elevations (in summer, nearly always above 8000 ft and often above 10,000 ft) in the Sierra Nevada & White Mountains. Puffy full white tail. Ears appear relatively shorter compared to L. californicus due to the large body. Turns white in winter, and often migrates to lower elevations.

Chipmunks

HB = Head+body length combined.

Note: these can be really difficult to ID when more than 2 species are present at a given locality. In some cases species-level ID is truly impossible with the current info we have, I try to spell that out below. I’d encourage more field investigation of vocalizations as they hold promise to sort out tricky IDs. Chipmunks molt and have summer & winter coats – I have relatively little experience with winter pelage, but can say that it is generally grayer and lower-contrast in all montane species, making them even more difficult to ID. This guide is meant for the summer coats. It’s also meant for wild, relatively undisturbed areas – chipmunk species in urbanized or disturbed (e.g. heavily logged, burned) sites seem to converge in pelage more & even occasionally hybridize, making ID extremely difficult (see Frare et al. 2016). Unlike other species, I’d avoid using iNat records as robust ID material for chipmunks as there are a ton of errors.

The best usage of this section of the guide is if you already have in mind a chipmunk or two as a possibility for your photo – whether that be based on range/habitat (field guide map) or coloration – and you can go to that species and learn which characteristics agree with your assessment or common similar species to watch out for. This is a record of my field experience for others to compare tentative ID’s and these notes cannot be promised to be error free. Also, I’ll be the first to admit that even clear photos of chipmunks regularly stump me – I have plenty of photos taking up space on my hard drive that I can’t ID to species in good conscience, so don’t be afraid to leave some at genus!

Chipmunks are only likely to be confused with Golden-mantled Ground Squirrel (Callospermophilus lateralis), which is larger, lacks facial stripes, and has a rich reddish-gold nape & shoulders.

1. Alpine Chipmunk (Neotamias alpinus) – smallest CA chipmunk. Endemic to alpine fellfields at or typically above treeline of C & S Sierra (2 disjunct populations – Yosemite area & Mt. Whitney/ Langley area), these days almost entirely above 10,000 ft. Pale yellowish gray with yellow sides & rusty brown dark dorsal stripes; outer dark stripe conspicuous. Tail has a mixture of yellow & black hairs with conspicuous black tip. Range differentiates from Least Chipmunk and small size, well-defined outer dark stripe, and black-tipped tail separates from Lodgepole Chipmunk as well as ecological differences (Lodgepole usually around trees, Alpine in treeless talus).

Left: Alpine Chipmunk with yellowish sides, rusty dorsal stripes with well-marked outer dark stripe, and black-tipped tail; Right: note rusty dorsal stripes & well-marked outer dark stripe

2. Least Chipmunk (N. minimus) – very small & slender; in CA usually associated with sagebrush in E part of state from Inyo Mts northward. Well marked dark brown to black dark dorsal stripes. Highly variable in coloration but I’m familiar with 2 morphs in CA – 1) fully dull grayish with little yellow on sides; 2) pale grayish fur with bright yellow sides. Some intermediates too. Often runs with long tail erect (other chipmunks do so less commonly) & quite terrestrial, preferring more open habitats than all other species except alpinus (allopatric). Often found in fully treeless sagebrush steppe. At times very difficult to separate from amoenus (especially morph #2), but generally more slender; the sides of amoenus are typically more orange, it has a more orange undertail, and the tail is relatively shorter (>HB in minimus, ~HB in amoenus).

Least Chipmunks – note super variable color phases! All have a well-marked outer dark stripe, long tails, and appear generally rather small and slender. The (rather confusing) bottom right individual was found at 10,000ft elevation in the White Mountains, where Least Chipmunk co-occurs with only the (rather different) Uinta & possibly Panamint Chipmunks – note how similar it is to Alpine Chipmunk.

3. Yellow-pine Chipmunk (N. amoenus) – small, brightly marked chipmunk of C & N Sierra Nevada, Cascades & Klamaths. Common in open, rocky yellow-pine forest; sunny montane conifer forest with pinemat manzanita; and especially aspen groves. Not much larger than least, but has a stockier look to it as its body is more robust and its tail length <= HB. All dark dorsal stripes well-marked, orange sides, conspicuous ochre-orange undertail. See N. minimus.

Left: Yellow-pine Chipmunk with bright orange sides, well-marked outer dark stripe, and tail about head-body length (not clearly longer); Right: Yellow-pine Chipmunk molting into winter coat (note how it is grayish & lower-contrast, making ID more challenging), though the well-marked outer dark stripe can still be appreciated.

4. Shadow Chipmunk (N. senex) – large, dull grayish chipmunk with a long tail and robust, blocky (even at times seemingly oversized) head. Usually in forests with dense understory of saplings or shrub cover. Very variable in color between populations. Individuals in humid coastal belt of NW CA (pacifica) are typically dark olive-brown in color with low-contrast dorsal stripes. Individuals in the inland population (N Sierra & Cascades) are brighter in coloration – grayish brown, with ochraceous sides. Dorsal stripes dark brown and fairly visible, including the outer dark stripe (cf. N. speciosus). Ears mid-length, and has medium-sized grayish white patches of fur behind ears; lower facial stripe does not significantly widen or darken below ear (cf. N. quadrimaculatus). Fur has a grayish cast to it; pale dorsal stripes are pale grayish rather than pure white. Inland NW CA populations are intermediate & variable in coloration; see N. sonomae. Note that sporadic hybridization with N. quadrimaculatus can complicate ID in the zone of sympatry in N Sierra.

Left: ‘coastal’ Shadow Chipmunk (N. s. pacifica); Right: ‘interior’ Shadow Chipmunk (N. s. senex). Note grayish pale stripes, brown dark dorsal stripes including a clearly visible outer dark stripe, blocky head, and smallish grayish white spots behind ears. Note the stark difference in pelage between the very dusky brown coastal form and the much paler, more contrasty grayish yellow inland form.

5. Siskiyou Chipmunk (N. siskiyou) – Large chipmunk that is externally identical to N. senex but of non-overlapping distribution in NW CA (siskiyou occurs N of the Klamath, while senex is to the S). Coastal population (N. siskiyou humboldti) identical to N. senex pacifica, likewise for interior populations.

Siskiyou Chipmunk is a dark, low-contrast species virtually identical externally to coastal forms of Shadow Chipmunk. It can only be separated by range (in CA, N of the Klamath River).

6. Yellow-cheeked Chipmunk (N. ochrogenys) – Large chipmunk that is identical to N. senex pacifica but of non- overlapping distribution (occurring S of the mainstem Eel River). Dusky olive-brown chipmunk with stripes often low-contrast. Occurs in shadowy understory of humid coastal belt of N CA.

Yellow-cheeked Chipmunk is another dark, low-contrast species virtually identical externally to coastal forms of Shadow Chipmunk. It can only be separated by range (S of the Eel River).

7. Sonoma Chipmunk (N. sonomae) – brightly colored chipmunk of chaparral & dry, open coniferous forests of N coast range. Inland populations have grayish brown thighs with deep orange sides; bright orange undertail is a good field mark. Main confusion is with inland populations of N. senex, which are typically darker and more brownish; perhaps best separated by call – vocalizations are quite different as sonomae have an audibly distinctive, high-pitched (almost birdlike) alarm call (Mammalian Species N. sonomae) very different from the typical, lower-pitched “chips” (3-5) you hear from senex (Mammalian Species N. senex). Also pronounced separation by habitat, with N. sonomae associated with dry, sunny lower-elevation black oak-ponderosa pine-montane chaparral (manzanita/ buckbrush) community while N. senex is found in darker montane conifer forests with more closed canopy and dense understory. Compared to N. amoenus, larger & longer-tailed with deeper orange sides & outer dark stripe paler brown; I’m also not sure these two overlap in habitat much, as N. sonomae is a lower-elevation form in NW CA. Largely excluded from coastal strip N of Bodega Bay by N. ochrogenys; coastal populations in Marin co. are darker brown (and the only chipmunk in this area).

Sonoma Chipmunk from inland Sonoma County, showing the bright orange undertail and striking orange sides well. I don’t have good photos of this species, so thanks to David Diller for letting me use his great photo here.

8. Long-eared Chipmunk (N. quadrimaculatus) – rather visually distinctive medium-large, brownish chipmunk with “rabbit ears” that are generally clearly longer than any other CA chipmunk. Two other great field marks are the big white furry spots behind the ears and the lower dark facial stripe, which widens to a broad blackish spot below the ear. Rich brownish sides & all dark dorsal stripes typically visible. A fixture of the lower montane mixed conifer forest belt of the C & N Sierra, especially common around sequoia groves in the S part of its range (e.g. around Yosemite NP). See N. senex.

Long-eared Chipmunks: in both photos, note the long ears, large white spots behind the ears, and lower dark facial stripe that widens into a darker blotch below the ear. A distinctive species.

9. Lodgepole Chipmunk (N. speciosus) – brightly colored medium-sized chipmunk of a variety of montane habitats, especially red fir & lodgepole pine forests; abundant S of Tahoe Basin, but comparatively uncommon to the N. Pale grayish thighs and bright orange sides. In my experience, the best field marks are 1) contrasty gray & orange coloration; 2) outer dark dorsal stripe is absent or very faint (cf. N. amoenus); and 3) outer pale dorsal stripe is bright white and wider than other dorsal stripes, particularly so in the San Gabriel & San Bernardino Mts populations (ssp. speciosus). Perhaps impossible to separate from Panamint Chipmunk in areas where their ranges approach (lower edges of eastside Jeffrey Pine forest in parts of Mono county), but the latter prefers drier, more open pinyon-juniper woodland. Perhaps impossible to separate from Uinta Chipmunk where ranges overlap (foxtail pine forest in S Sierra), though this species has sadly almost vanished from the Sierra these days.

Lodgepole Chipmunks: note contrasty coloration, very faint/absent outer dark dorsal stripe, and bright white outer pale dorsal stripe.

10. Uinta Chipmunk (N. umbrinus) – warm brownish chipmunk of bristlecone-limber pine forests, largely restricted to White-Inyo Mtns of E CA these days. Dark dorsal stripes are dark brown and the outer stripe is inconspicuous to absent. Overlaps in range (but not so much microhabitat) with much smaller & more slender Least Chipmunk. Panamint Chipmunk mostly occurs at lower elevations, but can be separated by more contrasting coloration & whitish face compared to the darker brownish Uinta.

Uinta Chipmunk: best identified by very faint outer dark stripe and brownish coloration within its range. This species is near-indistinguishable from (and once considered conspecific with) Lodgepole Chipmunk.

11. Panamint Chipmunk (N. panamintinus) – stunning but skittish & uncommon chipmunk of pinyon-juniper woodland in arid mountain ranges along the W edge of Mojave & Great Basin. Very sharply marked, with pale gray thighs contrasting with rich orangish-red flanks and rusty red dark dorsal stripes. Face very whitish, with facial stripes thin and pale brown. Can be very similar to Lodgepole Chipmunk but almost always found in lower, drier sites with sparse trees.

Although this photo is horrendous, it does illustrate the whitish face, pale grayish thighs & rich orange-red sides, and rusty red dark dorsal stripes (including an outer stripe) of the attractive and rather skittish Panamint Chipmunk. I’ll replace this with a better photo soon.

12. Merriam’s Chipmunk (N. merriami) – medium-sized grayish, usually dull, low contrast chipmunk of C & S CA. Prefers dense chaparral. Variable in coloration (some are brighter, with rather orangish sides) but pale dorsal stripes are grayish (unlike any other sympatric Neotamias), and in some populations (especially ssp. kernensis of SE Sierra) all dorsal stripes are very faded and indistinct. Thankfully the only species that is usually sympatric is Lodgepole. Coastal Santa Cruz Mtns population (ssp. pricei) is more saturated in color.

The poor photo on the left indicates a Merriam’s Chipmunk typical of inland CA, while the individual on the right is from the coastal (Santa Cruz Mtns) population. In both cases, note the low-contrast patterns and dull grayish pale dorsal stripe of this species.

13. California Chipmunk (N. obscurus) – very similar to N. merriami, especially the pale, grayish, low-contrast inland populations (e.g. ssp. kernensis). Externally indistinguishable from locally sympatric N. merriami (unless in hand when you can see the color of the hair bases of the lower neck & chest – black in obscurus and white in merriami everywhere except the San Bernardinos), so should be sought in sites where the latter is absent (e.g. around pinyons in Joshua Tree NP, the lower pinyon-juniper belt on the desert slope of the San Bernardino Mtns ex. Pioneertown or Cactus Flat, or above 7500 ft in the San Jacinto Mtns). These days, the only chipmunk at high elevations of Mt. San Jacinto as Lodgepole Chipmunk has seemingly been extirpated here. In San Jacinto Mts, the 2 species can be separated by call as N. obscurus lack the terminal pulse which is present in N. merriami (Blankenship 1987). Note that there do not seem to be consistent acoustic differences that permit separating this pair elsewhere, however. 

California Chipmunks: externally identical to Merriam’s & note the remarkable variation in pelage among this species even within a population (both individuals are from the same site!). The poorly-marked stripes and grayish pale dorsal stripe separates this species from Lodgepole Chipmunk in range.

Kangaroo Rats

We try to curate California iNaturalist k-rats so that iNat can be a reliable resource for range and species ID.

Kangaroo Rats: medium-large rodents with big, round heads and small, round ears that move by rapid bipedal hops; generally tan in color, with white hip stripes; long tails with lateral black and white stripes and a pronounced “flag” (crest) in the terminal ⅓ or so.

“Banner” refers to a thickly-furred black + white region at the tip of the tail.

“Four toed” kangaroo rats do not have a dewclaw. “Five toed” kangaroo rats have a small dewclaw visible in the hand or on roadkills (and very rarely in photos).

In the Great Basin, you may also find the strange & remarkable Kangaroo Mice (Microdipodops). They are not covered in this guide as they are quite distinctive, having huge “frog feet”, crestless slightly thickened tails (where they can deposit fat), an enormous head, large rounded ears surrounded by thick white fur, and a generally very round appearance.

1. California Kangaroo Rat (Dipodomys californicus) – medium sized, NorCal k-rat with obvious white tipped tail following black portion. A few very rare individuals lack the white tip! Usually fur is olivaceous. Only source of confusion is Desert k-rat in the Honey Lake area: Desert k-rats are paler (non-olivaceous), larger, and are usually associated with sand dunes.

California Kangaroo Rat: note white-tipped tail preceded by a black “pencil”

2. Desert K-rat (D. deserti) – large sized, pale k-rat that always has a pronounced white tail tip this is typically preceded by a dark grayish banner (sometimes the banner is paler gray, rather than blackish). Fur is a pale tan/sandy color. Paler than all other k-rats except some Merriam’s in SoCal low desert. Usually found in sand dunes or deposits of deep windblown sand, less commonly in large sandy washes. Presence readily visible by large burrows at shrub bases.

Desert Kangaroo Rat: note large, pale body, black “banner” & white tail tip, unique among k-rats within its distribution in CA (except perhaps a marginal D. californicus).

3. Giant K-rat (D. ingens) – HUGE k-rat, adults are impressively robust (especially if you’ve seen a lot of k-rats), but younger or smaller ingens may be confused for adult Heerman’s k-rats. Separating these two can be a challenge, but generally Heerman’s show more contrasting dark tail stripes: especially the lower stripe, which can be a bit dull in ingens. However, at high-abundance sites separating them is typically not an issue, as Giant k-rats live in massive colonies on level or gently rolling open grassland (readily identified by “precincts” – areas of clipped vegetation around burrow entrances) where they exclude the smaller Heerman’s.

Note the robust body of this very large k-rat species, as well as the 5th toe, the tiny pinkish dewclaw-like structure visible on the inside of the hind foot. Besides sheer size, this species can also be identified by its sign & the degree to which it engineers its ecosystem (precincts, etc.).

4. Heermann’s K-rat (D. heermanni) – Medium-sized, warm brown “broad-faced” k-rat with well-marked black tail stripes and convex, dome-shaped forehead. Ears can be large in coastal populations, but the outer edge is often somewhat angled (not wide & rounded). Widespread in central CA; habitats include coastal scrub, chaparral, oak woodland, arid grassland & alkaline scrub. It typically prefers scrubbier areas with more slope, especially where sympatric with Giant k-rat. See ingens, nitratoides, and venustus. Similar in appearance to the allopatric D. stephensi (illustrated below).
5. Fresno aka San Joaquin K-rat (D. nitratoides) – smallest central valley k-rat (comparable in size & appearance to allopatric Merriam’s, though slightly different in color). Typically it is pale grayish in color, and the tail stripes are rather dull, especially the lower stripe. It has a somewhat blunt nose in photos. It is 4-toed in hand (compared to 5-toed Heermann’s). Habitat is a good way to clue yourself into this species: it is typically associated with sandy washes with ephedra or saltbush. They seem to burrow around/under shrub cover, and therefore prefer light shrub cover on gently rolling terrain (scarcer on flats and steeper hills with heavier brush). See heermanni. 
6. Narrow-faced K-rat (D. venustus, incl. elephantinus) – coastal (venustus, sanctiluciae) and inland (elephantinus) populations are distinct in appearance, with the coastal animals typically a brownish olive color (these are the darkest CA k-rat except some populations of Agile & Dulzura) and inland animals grayish brown. Large and quite lanky in appearance, it is the 3rd largest CA k-rat after Giant and Desert. Good field marks include very large rounded ears with dark hairs (ears protrude visibly above the skull in elephantinus) and “narrow face” (skull evenly slopes downward between the eyes and does not bulge like in Heerman’s). The narrow face is well visible here. This species typically prefers tall, fairly closed, unburned chaparral with ceanothus and/or manzanita. It is uncommon and populations may be quite localized to the vicinity of stands of preferred plant species. See heermanni.

The ‘Elephant-eared’ Kangaroo Rat here (a subspecies of Narrow-faced Kangaroo Rat) is characterized by its huge, rounded ears, “narrow face” (not markedly bulging between the eyes), and grayish back. The coastal forms of this species are even darker grayish in color.

7. Agile K-rat (D. agilis) – “Narrow-faced” (head is triangular when viewed frontally & evenly sloping downward, without a bulge between eyes) k-rat with large round ears partially coated with dark hairs. Coastal animals are dark olive-gray in color and are generally similar to Narrow-faced in appearance (but are allopatric). Interior animals (Tehachapi Mts) are warmer brown, but typically show some olive tones, and can be very difficult to separate from Panamint (and Heermann’s, if sympatric) in this region. To help, note narrow face, more olivaceous coloration, and larger, rounder ears with a fairly conspicuous coating of dark hairs. See Panamint K-rat. 
8. Dulzura K-rat (D. simulans) – See agilis. Dulzura are a split from Agile (they are almost entirely allopatric). Dulzura are usually similar in appearance to coastal Agile in far S coastal CA. They can be a warmer brown color on the E slope of the peninsular range (e.g. Scissors Crossing area of SD co.), where they are scarce. There is a narrow region of overlap between Dulzura and Agile in the Jacinto badlands where they are not distinguishable. Five toes, size, dark coloration, and large ears separate Dulzura from the 4-toed coastal ssp. of D. merriami (parvus & collinus). Also see Stephens’s K-rat.

A Dulzura Kangaroo Rat from the Santa Rosa Mtns of Riverside Co. – note the round, large ears at least partially coated in blackish hairs; “narrow” (i.e. triangular) face; and olive-gray hues to the back. The coastal populations are even darker olive-gray vs. this inland animal.

9. Stephens’s K-rat (D. stephensi) – warm yellowish brown medium-sized k-rat generally quite reminiscent of Heermann’s. Has “broad face” (bulging between the eyes) and well-marked dark tail stripes. Where populations are dense, it seems to form localized colonies with obvious burrows characterized by clipped precincts joined by trails, to my eyes like a smaller version of Giant. Typically separated from Dulzura by habitat (Dulzura in dense chaparral and sage scrub, with Stephen’s a specialist of open gently rolling to level grassland, with sparse sage – especially sparse grazed or recently burned grassland, like the site where I took the below photos).

Stephens’s Kangaroo Rat is a classic “broad-faced” kangaroo rat with well-marked upper and lower tail stripes (like Heermann’s, not pictured).

Also (despite the photo being quite poor) note the smaller, narrower, and more pinkish ears of Stephens’s Kangaroo Rat compared to Dulzura Kangaroo Rat.

10. Merriam’s K-rat (D. merriami) – widespread small k-rat in all warm and hot deserts of CA on sand and gravel. It has 4 toes. The lower dark stripe on tail is usually not very distinct, and the tail has a long dusky tip. Coloration can vary (pale buffy to medium brown), but typically tan in color. Can be very pale in lowlands near Borrego springs, Carrizo gorge, and Imperial valley. Rare coastal-slope forms (collinus & parvus) of inland valleys W of the Peninsular Ranges (in SBD, RIV & SD counties) can be more yellowish gray in color. See ordii, panamintinus and microps.

Merriam’s Kangaroo Rats are typically small-bodied and have long dusky tail tips. The bottom individual is of the rare ssp. collinus of Peninsular Range valleys in W Riverside & inland San Diego counties.

11. Ord’s K-rat (D. ordii) – small k-rat found on sandy soils in Great Basin part of CA. It is 5-toed, so readily separable from Merriam’s in-hand, as roadkill, or even with a very lucky photo. Otherwise they are challenging to separate (“merriord’s”), but Ord’s k-rats seem to have more well-marked dark tail stripes (especially the lower stripe), have maybe a (at least relatively) shorter tail, and show a blunter facial profile with a quite short snout. Ord’s are said to outcompete Merriam’s on sandy soils, but I’ve seen roadkills of both species in sandy great basin desert almost side by side. Probably best sought in far NE CA or in Mono Lake NE shore, where D. merriami does not occur.

Ord’s Kangaroo Rat is a small k-rat externally very similar to Merriam’s – this individual was identified by range, found in an area of sand dunes NE of Mono Lake where D. merriami do not occur. Potentially the shorter snout & shorter tail brush may be field marks to separate this species, but this is currently unclear.

12. Panamint K-rat (D. panamintinus) – medium to large k-rat, often of sandy soils. Uses a range of habitats in Mojave & Great Basin from joshua tree and creosote desert, pinyon-juniper, sagebrush, and alkaline scrub on salt-encrusted playa margins. It has a lot of white marks on the face and a very pale, brownish white tail crest – when seen well, I find these markings & larger size the best fieldmarks to separate it from merriami or ordii. In addition, Panamintus is 5-toed vs the 4-toed merriami. In the Great Basin, panamintus seems to prefer higher-elevation sloping sites than ordii (e.g. Bodie Hills). See microps.

Panamint K-rat is a medium-sized k-rat especially common in the Mojave & Mono Basin. It is readily identified by a combo of size, its light whitish gray tail brush, and the large extent of white facial markings (vs. e.g. D. merriami).

13. Chisel-toothed K-rat (D. microps) – medium-sized k-rat found in high desert valleys typically on gravelly soils, but may also use sandy sites with saltbush. Seems to be a specialist of habitats dominated by blackbrush or saltbush in CA. Microps can be very difficult to separate from merriami and panamintinus when not in hand – best sought in sites where allopatric from panamintinus! Typically shows a sharply marked facial mask and “gunmetal gray” or olive hues on the back especially, thanks to a suffusion of darker hairs. Tail dusky. 5-toed. If in-hand, note chisel-shaped incisors. Perhaps the only CA k-rat that regularly climbs into bushes to forage (uses “chisel teeth” to scrape off the fleshy part of leaves).

Chisel-toothed Kangaroo Rat is challenging to identify, but in the right sites in CA needs to only be separated from D. merriami. Note the sharper facial mask, well-marked tail stripes, and the steel gray hues on the dorsal fur.

Pocket Mice

We try to curate California iNaturalist pocket mice so that iNat can be a reliable resource for range and species ID.

Distinguishing Chaetodipus and Perognathus

Coarse-haired Pocket Mice (Chaetodipus) are small to large mice that generally look like miniature kangaroo rats, but more slender and with elongated heads and relatively long ears; variable in color, always lacking hip and tail stripes. Their tails are well-haired and typically end in a crest. These sometimes hop, but generally move in quadrupedal (squirrel-like) bounds.

Soft-furred Pocket Mice (Perognathus) are generally smaller, and all have short, round ears with little white/yellow spots underneath them. Their tails are usually less hairy, and generally lack a pronounced crest. They are always heart-meltingly adorable.

In California, the only scope for confusion between Chaetodipus and Perognathus is Perognathus mollipilosus and the very similar P. alticolus. Interestingly, both are proposed to be grouped into a new subgenus (Cricetodipus) which is thought to be an intermediate between Chaetodipus and Perognathus. However, both species have less crested tails than Chaetodipus, and still possess rounded ears with white spots. They are larger, though, than all other Perognathus in California and have a more Chaetodipus GISS.

COARSE-HAIRED POCKET MICE (Chaetodipus)

I typically group Chaetodipus into 2 groups – “spiny” Chaetodipus, which show thick spine-like hairs, and “non-spiny” Chaetodipus which show a mix of thicker (often dark) hairs and long guard hairs (especially projecting from the rump), but never true “spines”.

Juveniles of all species can be difficult to separate especially in areas where multiple species occur (e.g. inland SoCal). Juveniles of the “spiny” group do not necessarily show spines, and are usually grayish (unlike the adults). Their proportions are a bit skinnier, though ear shape is fairly consistent (and what I would use a primary basis for ID of these individuals). Juveniles of the “non-spiny” group can be an absolute nightmare to ID and I often don’t even try. It’s worth noting that even adults in this group can be really difficult, especially in parts of the Colorado Desert (e.g. alluvial fans) where all 3 species often occur alongside each other.

1. California Pocket Mouse (C. californicus) – (usually) spiny, with white spines on rump & flanks; long ears that protrude visibly above the crown of the skull when erect (“rabbit ears”), and “crinkle” when folded. Adults often show an orange lateral line and white rump spines (& flanks, to an extent). These two features, however, are often missing in the juveniles. Tail is well crested.

This poor photo of a California Pocket Mouse (I hope to replace it soon!) from Pinnacles NP (where it is the only pocket mouse species) nonetheless shows 2 key features of the species – its orangish lateral line dividing the dorsal & ventral pelage; and its long ears that are somewhat pointed when folded. However, the white rump spines are not really visible.

2. San Diego Pocket Mouse (C. fallax) – spiny; oval-shaped or elliptical ears typically do not protrude markedly over the crown of the skull. Typically shows rump spines, but most are thin spines that are at least partially blackish – this color is best seen in close photos, and works well for separating the species from spinatus and californicus (when spines are present), as in my experience those two species have only white spines. Spines on hips are more whitish. 2 rather distinctive subspecies in CA:
   1. Northwestern S D PM (C. f. fallax) – usually medium brown in color with buffy lateral line; best differentiated from C. californicus by spine color & ear shape/length. These two species are typically allopatric in S CA, with californicus preferring montane chaparral (almost always the only Chaetodipus over 4000ft in S CA), except in some parts of SD & W RIV counties where they both use coastal sage scrub or chaparral.
   2. Pallid S D PM (C. f. pallidus) – pale grayish or grayish orange, with a faint orangish lateral line (often very inconspicuous). Best separated from C. spinatus (which has similar ear shape) by the extent of spines – in Spiny PM, these are denser and extend laterally all the way to the shoulders of the animal. Also, the Spiny PM is typically a darker brown in color. Typically found in or near pinyon and/or juniper.

This Pallid SD PM from arid, rocky juniper habitat near Anza-Borrego SP breaks my rule of not using juveniles here. Note that its hip spines are quite thin and haven’t really come in yet. However, it does illustrate the oval ear shape of this species quite well (though note that some individuals of the coastal population can have slightly longer, if still fairly round ears) as well as the faint orangish lateral line.

I expressly sought to avoid using roadkill photos here, but this is the only photo I have that shows the characteristic black spines that can be typically found densely in the rump pelage of C. fallax (C. f. fallax in this case) alongside thin whitish spines near the hips. This individual was found at a site where C. californicus is not known to occur despite extensive trapping surveys.

3. Spiny Pocket Mouse (C. spinatus) – spiny and fairly small, with dense white spines that extend to shoulders. Ears are relatively short and oval-shaped (like C. fallax). Usually prefers hot, low-elevation habitats like the vicinity of fan palm oases & rocky slopes with ocotillos.

This Spiny PM shows the relatively short, rounded ears and the thick white spines on the rump & (well up the) flanks that characterize the species.

4. Long-tailed Pocket Mouse (C. formosus) – not spiny; mid-sized; fur smooth and often shiny, variable in color, grayish and warm brown phases can occur in the same sites. Juveniles similar in color to adults. Usually have a luxurious tail crest of quite long hairs. Often has remarkably long ears that project well over the crown of the skull; when folded, ears appear “arrow-tipped” or elf-like. Widespread in rocky or gravelly habitats throughout the Colorado, Mojave, Great Basin deserts – true survivors, they can be abundant in sites with next to no vegetation.

Long-tailed PM has the longest ears of any CA desert pocket mouse – often almost comically long, as in this individual.

This Long-tailed Pocket Mouse illustrates the “arrow-tipped”, elf-like look of this species’ ears when folded.

Note that some Long-tailed Pocket Mice have relatively short ears, albeit still pointed, as in this individual from Anza-Borrego. While ear shape made ID more challenging than usual, microhabitat (rocky slope away from areas of sand) and its thick tail crest of long hairs made me conclude Long-tailed PM.

5. Desert Pocket Mouse (C. penicillatus) – not spiny; small & slender Chaetodipus that is locally abundant in hot, sandy low desert with creosote and mesquite, especially in SE CA. Usually pinkish tan in color, or pale grayish brown. Juveniles pinkish brown. Rump fur often shines. Ears are triangular, rounded, relatively short and generally do not protrude over the crown of the skull. Tail crest hairs typically short. This species (especially juveniles) may be confused with Perognathus, but note more crested tail and less rounded ears. Large individuals may be confused with C. rudinoris and longer-eared individuals with C. formosus – sometimes all 3 species can be impossible to separate! Habitat is a good guide though, where sympatric in SE CA.

This Desert Pocket Mouse from very sandy low desert shows the typical features of medium-length wedge-shaped ears, small & slender body, and relatively short hairs in its tail crest.

6. Baja Pocket Mouse (C. rudinoris) – largest Chaetodipus in CA (a split from C. baileyi), structurally like a large, very robust version of C. penicillatus. Fur yellowish gray and coarse-looking. Pelage color varies depending on soil (seemingly paler in sites like Anza-Borrego & Deep Canyon, and often darker with more olive/blackish grizzle in desert pavement sites further E), but you often see mixed yellow-gray and blackish/olive hairs of varying thickness on the head & rump, and sometimes even longer guard hairs in adults. Juveniles often pale yellowish brown. Ears fairly short & rounded and tail crest reduced compared to C. formosus (which can be very difficult to separate). I often ID these by robust proportions of head and body (especially compared to C. penicillatus) – when seen at a distance, it can even recall a small k-rat. The tail appears thickened, especially in its first 1/3rd. In CA, typically associated with soils with a heavy gravel component; most records from stony desert pavement composed of dark-colored rock chips or bajadas at the base of the peninsular range (often with jojoba). In my experience, most common in the Desert Center area of Riverside Co.

This Baja Pocket Mouse from a “desert pavement” of dark stones shows the typical yellowish gray pelage, speckled with darker olive & gray hairs as well as the rounded, wedge-shaped ears that do not protrude clearly above the skull (cf. C. formosus). Thanks to Jon Hall (mammalwatching.com) for sharing this image.

This Baja Pocket Mouse, seemingly recovering from a recent injury, was impressively robust in the field (my first thought was small Merriam’s K-rat when I saw its body from a distance!). Note the speckled yellowish gray coloration and rather thick tail (particularly noticeable in the field when seen alongside its large body), as well as the diminished tail crest (vs. C. formosus).

SOFT-FURRED POCKET MICE (Perognathus)

Small to tiny round mice with large round heads and small round ears and round ears with white spots underneath. Tails lack a pronounced crest. No hip or tail stripes. Fairly short-limbed. Locomotion is somewhat odd, an erratic slow-speed quadrupedal bound in which animals hop with hind feet, touching down with front feet for balance. Frequently changes direction by hopping in place. Locomotion of ‘Cricetodipus’ is again, a bit more Chaetodipus-like, faster and seemingly less erratic at times.

1. Little Pocket Mouse (P. longimembris) – tiny and rather slender in proportions. Smallest CA pocket mouse. Fur is silky & shiny. For a Perognathus, the tail is relatively long, pinkish, and sparsely haired. Usually grayish in color, varying from a darker gray in coastal populations (cantwelli, brevinasus/internationalis) but inland populations on gypsum sands can be creamy white in color (arenicola, bangsi). Often associated with sage in sandy washes.

Note the grayish fur, tiny body, and long pinkish tail of the Little PM.

2. San Joaquin Pocket Mouse (P. inornatus) – a small, silky species that is not much bigger than P. longimembris, but the body is stout, making it seem more robust. Tail is usually proportionally shorter than that of P. longimembris (though some individuals of the W population show a slightly pencilled tail). Fur is warm orangish gray or brown and shows an orangish lateral line. Typically allopatric from P. longimembris except in the W Mojave desert, where identified by proportions (body & tail) and lateral line.

Note the warmer-colored fur and orangish lateral line typical of the San Joaquin PM.

3. Great Basin Pocket Mouse (P. mollipilosus) – adults are distinguishable by large size (for a Perognathus). Juveniles may overlap in size with smaller Perognathus, but all individuals of this species show a lobed antitragus (= flap of cartilage in front of the ear canal) in the ear, which readily identifies this species & is well visible in photos. Yellowish spot below the ear. Tail shows a pencil of short/medium-length hairs on the terminal third, especially when viewed closely. Coloration varies from grayish to dark olive.

This young Great Basin PM initially recalled a smaller Perognathus due to proportions and sparsely-haired tail, but was unambiguously identified by its prominently lobed antitragus.

This adult Great Basin PM shows the large size and dark olive fur typical of the species well. Note the antitragal lobe and the well-haired tail, unique among Perognathus (along with the similar P. alticolus).

4. White-eared Pocket Mouse (P. alticolus) – rare, highly localized large species that is near-identical to Great Basin PM but allopatric. Paler in color and ear spots are whitish rather than yellowish. Only potential confusion is with P. inornatus (locally sympatric in the Tehachapi Mts.), but haired tail with conspicuous pencil (usually), lobed antitragus, and larger proportions should separate it. The San Bernardino Mts. population has seemingly vanished.

This (brightened) photo of a juvenile ‘Tehachapi’ White-eared Pocket Mouse is not great for showing identification features, but I’m including it as it’s one of a handful of photos that exist of this very rare & elusive species! Those with a keen eye will note the antitragal lobe in the right ear.

Woodrats

Medium to large “rats” with broad snouts, white bellies and hands, and long at least partially furred tails. All CA species construct nests from sticks and other debris of varying degrees of complexity, ranging from large 4-ft tall “huts” of N. fuscipes and macrotis to pine cones or sticks crudely stuffed into rock crevices as in N. cinerea or N. lepida. (May be confused with Rattus, which have narrower snouts, uniform grayish brown dorsal & ventral fur, and naked scaly tails.)

1. Bushy-tailed Woodrat (N. cinerea) – Readily ID’d by thickly furred, bushy tail that broadens at tip; tail usually gray or brownish above and white below. Largest CA woodrat, but note that there’s a fair bit of color & size variation in this species; the NE CA individuals (ssp. occidentalis) are HUGE and gray with very large tails, while those in Mono & Inyo counties (ssp. acraia) are smaller, with thinner tails and are pale grayish to yellowish brown in color. Note that there is scope for confusion with the smaller N. lepida with the latter; although the tail may be well-furred in lepida, it never shows bushy fur that widens to the tip. Mostly associated with rocky outcrops or cliffs. Sticks nests typically crude arrangements of pinecones, sticks, or pebbles stuffed into rock crevices; outcrops occupied by N. cinerea typically show areas of whitewash & scats.
2. White-throated Woodrat (N. albigula) – fairly large, large-eared woodrat in low-elevation parts of the Colorado Desert in far SE CA (E Riverside, far SE San Diego & Imperial counties) where it is fairly localized to the desert wash vegetation community of tall mesquite & ironwood (Olneya) trees. With the exception of large individuals (which are safely identified by size alone), can be difficult to separate from N. bryanti/lepida when not in hand (since the white bases of the throat hairs cannot be seen), but in CA N. albigula is usually rather larger and has larger ears.

I don’t have any halfway decent photos of the White-throated Woodrats I’ve seen in CA, so thanks to Jon Hall (mammalwatching.com) for sharing this image. This individual was photographed at the same location as the Desert Woodrat below – note the larger body & especially ears and darker tones to the fur.

3. Bryant’s Woodrat (N. bryanti) – small, grayish woodrat of rocky chaparral & sage scrub in C & S coastal CA. Very similar to N. lepida (split from) but usually darker grayish and slightly larger (at times substantially so). Local zones of sympatry & hybridization in mountain ranges on the W margin of the Mojave & Colorado deserts (e.g. Piute Mtns, Whitewater area of San Gorgonio Pass, etc.) where species-level ID is impossible. In CA, only ssp. intermedia is found. See N. fuscipes.
4. Desert Woodrat (N. lepida) – small, usually pale grayish brown woodrat that is widespread in most CA desert ecosystems, especially on gravelly and rocky soils. Note that this species can show a fair bit of variation in size! Sticks nests are crude piles of vegetation or even rock chips (in desert pavement areas) at the base of shrubs. Much smaller than other sympatric Neotoma. Color varies, usually pale yellowish gray (but dark gray in N of range, and melanic with dark grayish upperparts and yellowish belly in lava flows of the Mojave). Hind feet white. Tail relatively short, bicolored & well furred. See N. c. acraia.

Desert Woodrat is a small, usually pale colored species with a relatively short, bicolored tail densely covered in short fur.

5. Dusky-footed Woodrat (N. fuscipes) – Very large woodrat of oak woodland, tall chaparral & mixed conifer forests of N & C CA. Typically warm grayish brown in color with white belly, tail mostly dark and sparsely furred. Hind feet grayish, not white. N. bryanti/lepida has a shorter, more bicolored tail & white hind feet. Builds huge (to 4 ft tall) stick nests that are a conspicuous feature of the landscape where this species is common. Very rare ssp. riparia in riverine forests of the Delta. See N. macrotis & discussion on distributions.

Dusky-footed Woodrat is a very large, warm brown woodrat with a weakly bicolored tail sparsely furred with dark hairs. The grayish hind feet are not visible in this poor photo (something I hope to fix soon!).

6. Large-eared Woodrat (N. macrotis) – Externally identical to Dusky-footed but allopatric in similar habitats of C & S CA. Widespread in coastal & montane S CA, with an outlying pop. in the Granite Mts (Mojave Desert); in C CA, found in the Santa Lucia Mts. and W slope of the Sierra Nevada north to S Fork American River, with Dusky-footed Woodrat elsewhere in the region. Very locally on the far SE slope of the Sierra Nevada in Kern & Inyo counties, in low riparian canyons. Hybridizes with N. fuscipes along the Nacimiento River at the Monterey-SLO county line.

Deer Mice (Peromyscus)

The stereotypical “Mickey” mouse with a pointed snout & large, sparsely furred ears. Fur appears soft & silky. Smaller and much more slender (especially the head) compared to Neotoma. They have larger ears and eyes than Mus, and are also more markedly bicolored between the brownish dorsal and white ventral coloration – hence their common name “deer mouse” (whereas in Mus, it grades gradually to a pale grayish belly). In contrast to Peromyscus, Reithrodontomys are smaller and more slender, with relatively longer tails and shorter, sparsely furred ears usually with distinct orange hairs at the base. These can be separated from Mus by their white bellies, shorter ears, less pointed snout, and haired, bicolored tails. Grasshopper Mice (Onychomys) can be very similar to Peromyscus gambelii/sonoriensis, but usually have blunter, wider snouts and shorter tails (approx. ½ HB in O. torridus and even shorter in O. leucogaster) that have a blunt, usually fully whitish tip (as opposed to bicolored to the end and tapering, as in Peromyscus). Jumping Mice (Zapus) may be encountered in the Sierra, Cascade & Klamath mountains as well as N coastal CA; they have very long, thin tails and long hind feet and usually show a distinctive two-tone dorsal coloration with a brownish back and yellowish sides. When moving at high speeds, Peromyscus can appear to hop but never use the spectacular 2+ ft high vertical jumps, landing on their back feet, as jumping mice do. Juveniles of all Peromyscus are grayish and very difficult to identify to species-level.

1. Gambel’s Deer Mouse (Peromyscus gambelii) – small Peromyscus with a short (≤ head-body length), well-furred, sharply bicolored (top blackish + bottom pure white) tail. Split from P. maniculatus. Body color is usually warm brown (juveniles grayish), sharply contrasting with a white belly with no lateral line. Brown fur often shows a slightly darker “saddle” along the back when compared to the sides. Ears are relatively short for the genus. Externally indistinguishable from P. sonoriensis, but distribution mostly non-overlapping. Precise range limits unclear, but widespread in C & S CA (roughly S of the SF Bay & Lake Tahoe), extending to the E slope of the Sierra Nevada and Peninsular Range (where it co-occurs locally with P. sonoriensis). Coloration & tail length can be rather variable based on locality, with individuals from wetter, more vegetated habitats typically being darker in color and longer-tailed. The dominant Peromyscus in areas with sandy soils, especially in flat sites with sparse vegetation (where it can be abundant); less common in rocky, brushy sites where often outcompeted by other Peromyscus. Terrestrial & scansorial. See P. sonoriensis and Onychomys (above).

Note the sharp boundary between the dorsal and ventral fur of this Gambel’s Deer Mouse, the “saddle” of subtly darker fur along the upper back (a feature of Peromyscus in the maniculatus species group), and the fairly short (~HB) well-furred and strongly bicolored tail. This species also has relatively short ears compared to other CA deer mice.

2. Western Deer Mouse (Peromyscus sonoriensis) – externally very similar to P. gambelii (see above), but of largely non-overlapping distribution. Split from P. maniculatus. Widespread in most of CA N of the SF Bay & Lake Tahoe, and also in the Great Basin, Mojave, and Colorado Deserts. Variable in coloration & tail length. Individuals in the humid coastal region of N CA are typically medium to dark brown with relatively long (= HB) tails, while those in the Great Basin & Mojave are often warm brown or yellowish with short (often close to ½ HB) tails. Scarce in the Colorado Desert (= ‘Desert’ Deer Mouse), where dark grayish brown and typically associated with large sandy washes (e.g. Snow Creek, San Felipe Narrows) – this population is taxonomically uncertain & may represent either P. gambelii or P. sonoriensis, but I’m grouping it with Western Deer Mouse for now based on similar habitat use. Juveniles are grayish, growing more brownish/yellowish with age. Most abundant on sand, and can enter very rocky canyon areas via washes. See P. gambelii & Onychomys.

LEFT: Western Deer Mice of the Mojave & Great Basin are usually a warm yellowish color, with a fairly indistinct “saddle”. RIGHT: an adult ‘Desert’ Deer Mouse from the Colorado Desert – this form is typically medium brown or grayish brown in color (surprising for adults, which are paler in other arid zones) and doesn’t show a clear saddle. Note however, despite pelage variation, both individuals show the typical short ears, sharp delineation between dorsal and ventral pelage (without a lateral line), and short, well-furred, bicolored tail.

LEFT: Western Deer Mice of north coastal CA (formerly P. maniculatus rubidus) are typically medium to dark brown or grayish brown (rarely yellowish), with relatively long tails (~HB length vs. < HB length in dryland populations). Otherwise typical of Western Deer Mouse with relatively short ears, “saddle”, sharp delineation between dorsal & ventral pelage (without a lateral line), and strongly bicolored well-furred tail. RIGHT: this juvenile P. sonoriensis is molting into its fulvous adult fur, but still shows grayish pelage typical of young Peromyscus.

3. Canyon Mouse (Peromyscus crinitus) – small, large-eared Peromyscus with a long tail (> HB). Best identified by tail, which is somewhat bicolored, clearly tufted, and covered in long hairs, but thinly enough that the underlying scaly skin can be seen. The only Peromyscus that appears to have a “crested” tail, as a result. Coloration usually yellowish brown (seemingly darker in Colorado Desert) with a white belly and indistinct yellow lateral line. Prefers steep, bare rock with sparse (at times even nearly no) vegetation cover. Can be similar to P. truei (which is usually larger, more grayish, and shorter-tailed where distributions overlap) but generally prefers hotter, lower-elevation sites below the pinyon-juniper zone.

Canyon Mouse is a small-bodied Peromyscus, usually with rather pale yellowish brown fur & large ears. The key “field mark” is the long (> HB) tail covered with fairly long hairs, especially at the tip where they lengthen to a small crest. Note the thin & indistinct ochre “lateral line” dividing dorsal & ventral pelage.

4. Pinyon Mouse (Peromyscus truei) – medium-sized, huge-eared Peromyscus with a bicolored, furred tail. Tail length & ear size vary, with individuals on the Pacific slope (W of the crest of Sierra Nevada and Transverse/Peninsular Ranges) having long (> HB) tails and large ears while those E of the Sierra crest have enormous ears and relatively short (< HB) tails. Ears shortest in coastal NW CA, where best separated from P. sonoriensis by long (> HB), bicolored, tufted tail. Coloration variable, but usually grayish with a rather thin yellowish lateral line; populations of coastal C & N CA are more darkly-colored. In coastal CA, care needs to be taken to avoid confusion with P. boylii (shorter-eared, with a brighter orange lateral line and more sparsely-haired tail) and P. californicus (larger & more orangish in color). Nearly always prefers steep rocky habitats, especially with chaparral or sparse woodland (e.g. pinyon-juniper).

LEFT: This Pinyon Mouse from interior CA (the SE slope of the Sierra) shows the typical huge ears & clearly bicolored, furred tail that distinguish the space, as well as the typically faint, yellowish lateral line. RIGHT: Pinyon Mice from the Coast Ranges, like those in the interior, have larger ears than other sympatric deer mice, but have more blackish tones to the fur than inland animals.

5. Brush Mouse (Peromyscus boylii) – medium-sized Peromyscus with medium-large ears. Tail long (usually > HB); tail pattern varies across its large range, but in CA I find it often sparsely-furred (especially the first half), weakly bicolored, and ending in a tuft of hairs. Hairs are short and thin, leaving pinkish gray scales visible, and the tail shows weak bicoloration. Dorsal fur is grayish and belly white, separated by a broad, bright orange lateral line – typically my best field mark for this species. Widespread in most CA mountains (exc. the Santa Cruz mountains) where usually associated with steep, rocky slopes; in coastal CA, I generally find them in live oak woodland (separating them from the more chaparral- associated P. truei) and occasionally rocky cliffs with sparse grass or herbaceous cover. Very abundant on the W slope of the Sierra on rocky slopes in lower montane forest (especially among Canyon Live Oak). An excellent climber, and probably the most arboreal of our Peromyscus (the only species regularly found 10-15ft up in the branches of live oaks). See P. truei, P. californicus, and P. eremicus/fraterculus.

LEFT: this Brush Mouse (P. b. rowleyi) from NW Mexico clearly shows the rich orange lateral line separating the white belly from the darker grayish brown dorsal fur, medium-length ears, and long tail ending in a tuft of longer hairs. RIGHT: Brush Mice of CA’s Pacific slope (P. b. boylii) are generally similar in appearance, sharing the orange lateral line & medium-length ears, but seem more grayish.

6. California Mouse (Peromyscus californicus) – large (by CA standards) Peromyscus with large ears, The largest Peromyscus species in the US in fact. Tail usually HB or longer, tufted, bicolored, and moderately well-haired (though some skin is still visible). Dorsal coloration is medium brown, with broad orangish sides and a pale belly. Often abundant in chaparral among stands of hollyleaf cherry and in the dense undergrowth of poison oak in live oak woodlands (or in the Santa Cruz Mountains, Redwood/Douglas- fir/Tanoak forest) where it occurs in & near the nests of Neotoma fuscipes or N. macrotis (hence its old name, “Parasitic Mouse”). In some areas (especially coastal S CA), can be very difficult to distinguish from P. boylii but usually told by larger size, darker dorsal coloration, more distinct tail bicoloration, and different habitat preferences.

California Mouse is a unique, large species mostly endemic to lower elevations in C & S CA. It’s fairly distinctive, with quite large ears, orange sides and a medium-brown back.

7. Cactus Mouse (Peromyscus eremicus) – medium-sized Peromyscus with medium-length ears and long (> HB) tail. Tail is often pinkish, slightly darker gray on top, and sparsely-haired to naked (at times bearing a diminished terminal tuft of small hairs), and the annulations in the skin are usually well visible in photos. I’ve noticed that populations on the E margin of the peninsular in Anza-Borrego SP often have more hairy tails than usual for the species. Dorsal coloration yellowish brown, with ochraceous lateral line and a pale belly. The head often has a distinctly grayish color compared to the body. Widespread & abundant in the Colorado Desert & E part of Mojave Desert, especially xerophytic scrub on rocky soils or among mesquite (climbing into the branches to feed); I don’t find this species as much in the flatter, sandy creosote-dominated areas, however. Some sources suggest that activity is greatest in early spring and reduced in the summer heat, though I’ve found them common into early July. Can be difficult to separate from Canyon & Brush Mice; the former usually shows a less distinct lateral line and longer hairs along the length of the tail. The latter often has a more furred, bicolored tail & mostly occurs at higher elevations, but may narrowly overlap on the E slope of the Peninsular Range. Both species usually show clearly long hairs at the end of the tail that form a more distinct tuft. Neither species has a grayish head.

This Cactus Mouse from low desert in NE Mexico shows the slightly contrasting grayish head, ochraceous lateral line, and pinkish tail covered with sparse hairs. Note that the tail of this species is often far from “near-naked” (as it is often describe in field guides), typically covered in short bristles that may lengthen to a small tuft at the tail tip, as visible in this individual.

Cactus Mouse foraging on seed pods in the branches of a mesquite in the Anza-Borrego Desert. Note the typical distinct ochraceous lateral line & grayish head. Unlike many other populations of P. eremicus, some Cactus Mice in this area often seem to have more densely haired tails that are covered in short bristles.

8. Northern Baja Mouse (Peromyscus fraterculus) – externally very similar to (and split from) P. eremicus but usually slightly darker in color. Locally abundant in chaparral (most common < 4000 ft) and sage scrub in coastal southern CA, roughly from the Santa Monica mountains southward. Systematics & range limits vs. allopatric P. eremicus are very unclear in E San Diego & Riverside counties; this species likely extends to the eastern slope at least as far as chaparral is present.

Bats

Bats are a fascinating group, but they can be very difficult to find & identify. I won’t cover bat-finding here, but will focus on bat identification. This can be a tricky process, and often you need to integrate multiple lines of evidence, usually photos (for external field marks), sonograms (of flight calls, not social calls etc.), and where available published data on species abundance. While I’ve done a fair bit of bat work in & out of the western US and consulted with biologists regularly regarding species ID, I won’t pretend to be an expert on either morphological field marks or echolocation! And this is despite spending a lot of time observing CA’s bats. Thus, I ought to emphasize that these notes cannot necessarily be depended upon as a standalone resource for certain IDs in this group, especially for difficult species e.g. genus Myotis. Even the scientists sometimes have to make best guesses. But to the best of my knowledge, everything here is accurate and should at least point you in the right direction!

More detailed data on FLIGHT CALLS can be found via Sonobat (http://sonobat.com/download/Acoustic_Features_of_Western_US_Bats_2018.pdf). Note that calls can often reduce the set of possibilities to 2-3 species, but distinguishing between these can be very difficult without close, clear sonograms recorded of a bat flying in an uncluttered environment and a fair bit of luck to get a diagnostic call type! There’s a fair bit of vocabulary here (f_c, CF, FM, etc) that I’m going to keep to a minimum in my notes to improve readability, but definitely if you are serious about observing CA’s bats what I’ve written should not be considered a standalone resource and I’d encourage you to explore deeper! Please note that although the Auto ID feature included in the Echo Meter series can be directionally accurate, I generally do not trust it and manually ID calls on my own (I’ve had calls from the same bat making multiple passes identified as 3 different species in the past!). Younger people can generally hear bats better.

PHYLLOSTOMIDS (leaf-nosed bats) – these mostly Neotropical bats are distinguished by the nose leaf (a fleshy projection from the nose). Only one species is resident in CA.

1. California Leaf-nosed Bat (Macrotus californicus) – large, distinctive, pale grayish bat with long rounded ears found in the Colorado Desert (especially Lower Colorado River Valley) and rarely the Peninsular Ranges of far S CA. Only potentially confused with Pallid Bat but readily separated by facial shape, especially presence of a nose leaf.

The California Leaf-nosed Bat is unique & distinctive among CA bats, with large rounded ears, relatively large eyes, and a noseleaf. I lost my only photos of this species due to card issues, so thanks to Jon Hall (mammalwatching.com) for sharing this image.

Two migratory, nectar-feeding species have been rarely recorded in CA: Mexican Long-tongued Bat (Choeronycteris mexicana) and (even more rarely) Lesser Long-nosed Bat (Leptonycteris yerbabuenae). Mexican Long-tongued Bat is mostly recorded in coastal San Diego County in fall & winter. If you happen to see these using a hummingbird feeder, put it up on iNaturalist! Choeronycteris has a longer snout and a U-shaped tail membrane, while Leptonycteris has a diminished, very narrow V-shaped tail membrane (basically a narrow band along each hind leg).

MOLOSSIDS (Free-tailed Bats) – a narrow, rat-like tail protrudes from the tail membrane (uropatagium). CA species have “dog-like” faces. With the exception of Mexican Free-tailed Bat, very unlikely to be seen roosting:

1. Western Bonneted Bat (Eumops perotis) – largest CA bat. Very recognizable in flight with its loud flight calls that sound like harsh metallic chirps and fast, straight flight. Produces long, fairly flat calls that are among the lowest-frequency of any CA bat, usually at or below 10 kHz and rarely above 12 kHz. Long, narrow wings have a huge, nearly 2ft (20-23”) wingspan! Very unlikely to be observed roosting unless sick or injured. Unmistakable if seen perched, with huge sweeping ears joined above the forehead; upper lip not wrinkled. Small colonies (often <10 individuals) day-roost deep within rocky crags or cracks high up on steep, rocky cliffs. Long-distance forager that seemingly does not night roost.
2. Big Free-tailed Bat (Nyctinomops macrotis) – another rather large species with audible calls, long wings (~16” wingspan and thus clearly larger than Tadarida/N. femorosaccus), and fast, straight flight. Metallic chirps are readily audible (for me) but sound softer and less harsh than those of E. perotis. Roosting situation similar to Eumops, and thus unlikely to be seen unless in flight; has a wrinkled upper lip & ears joined above the forehead. Produces long calls (often fairly flat); range of recorded frequencies for this species seems rather large, from 12-25 kHz but usually 14-18 kHz (Naver & Gore 2001). My confirmed records from AZ are flat calls mostly at 14-15 kHz (and my likely ones from LA county at ~16 kHz). 12-14 kHz calls may be confused with E. perotis, but the main source of confusion is with N. femorosaccus which may overlap extensively in call parameters with this species. Parameters to distinguish these species confidently in CA remain unclear, though N. macrotis calls may run overall lower (usually 14-18 kHz vs. 17-21 kHz). A rare species in CA, and most records are associated with migrants in the late fall/early winter in sites on the coastal slope of S CA e.g. Pt. Loma near San Diego and low-elevation, coast-facing canyons of the San Gabriel Mountains. Very rare as a breeder, with a few confirmed summer records in coastal SD co. & possibly at Barker Dam in Joshua Tree NP. However, may be expected anywhere in the southern ⅔ of CA (acoustic records reported from Yosemite Valley).
3. Pocketed Free-tailed Bat (Nyctinomops femorosaccus) – smallish free-tailed bat (wingspan ~13”) only slightly larger than Tadarida, to which it appears generally similar in flight. Calls should be audible for young women, but are at the upper edge of my hearing range (so Nicole hears them a lot more easily than I do!). Calls are similar to N. macrotis but slightly higher; my recordings are mostly 17-20 kHz (in line with the NA Bat Monitoring Group’s data). Pretty widespread & common in W Riverside & San Diego counties (FAR more so than N. macrotis, and thus the expected Nyctinomops in most of far S CA), both on the coastal slope and in Anza-Borrego desert, where they are often found around palm oases. Potentially occurs further N in LA & San Bernardino counties, but I’ve yet to read of a confirmed (specimen) record. If seen roosting, can be separated from Tadarida by ears joined above the forehead (they are not in Mexican Free-tailed Bats) and from N. macrotis by size.
4. Mexican Free-tailed Bat (Tadarida brasiliensis) – smallish free-tailed bat (wingspan 12”) that is widespread & abundant in most low-lying (mostly below 4000 ft) parts of CA, except in the NW. Flight calls are highly variable and above the hearing range of most people, except for call types that are uncommon in open-air flight (they are quite loud around roosts, however!); the most identifiable are the typical long, relatively flat or gently sloping calls that usually range from 22-28 kHz and thus higher than Nyctinomops. But there are also a range of social calls, feeding buzzes, etc. etc. that may show up on your detector. Doglike face with large, round ears and wrinkled upper lip; naked tail protrudes from tail membrane (hence “free-tailed”). Famous for mass emergences (in CA, the largest colony is near Davis). Also often seen roosting in anthropogenic structures such as expansion joints of bridges.

These Mexican Free-tailed Bats in a bridge expansion joint – a typical roosting situation – clearly show the characteristic wrinkled upper lip of Tadarida and Nyctinomops.

Note the rat-like “free tail” that characterizes molossids (free-tailed bats) like this Mexican Free-tailed Bat.

MYOTIS (mouse-eared bats): small bats with “mouse-like” pointed ears (hence their name) and plain faces with a pointed snout. The genus is identified by its long pointed tragus. Can be very difficult to identify to species level! Most of these species are active early in the night, just after sunset, and can be regularly seen night-roosting under bridges. Big Brown Bat (Eptesicus fuscus) is superficially similar, but can be distinguished by larger size, often longer fur, and a large, seemingly bulbous/inflated nose. Canyon Bat (Parastrellus hesperus) can appear very similar to Myotis californicus and M. ciliolabrum, but has a short, blunt (“club-shaped”) tragus and a shorter, blunter snout. Keeled calcar = a small projection of skin extending from the ankle.

1. California Myotis (Myotis californicus) – small myotis with short to medium-length ears. I find the fur to often be frizzy and dull (not shiny or glossy); the color is very variable (even within populations), ranging from pale yellowish or whitish orange in the desert to rich chestnut brown in forested N CA, with the belly often slightly paler than the back. In much of CA, usually orange or orangish brown. Facial skin usually brownish, but can be dark brown to blackish. Desert forms superficially very similar to M. ciliolabrum, and at times not separable (even in hand) without acoustics; but usually pelage (orange) & facial skin color (brownish) usually markedly different elsewhere in CA where the 2 species are sympatric. Keeled calcar. Calls approx. 50 kHz & end with a downward pointing tail (which distinguishes this species from M. yumanensis); behavior can also be a good clue (californicus often forages under a woodland canopy, while yumanensis skims the surface of open water). Widespread throughout CA. Most active soon after sunset.

These California Myotis (a mother & baby) were roosting in a park bathroom in Central CA – call recordings confirmed the species. Note the typical dull orangish fur, relatively low-contrast brownish facial mask & medium-length ears. The thin tragus with a pointed tip identifies these bats within genus Myotis.

LEFT: A California Myotis from the southern Sierra, with dull, frizzy orange-brown fur (with little contrast between dorsal & ventral fur) and brown facial mask. RIGHT: Animals from forested parts of N CA have darker facial skin & chestnut-colored fur but share the dull fur texture & low-contrast mask. I don’t have photos from S CA deserts yet, but M. californicus here is pale whitish orange with dark brown facial skin.

2. Western Small-footed Myotis (Myotis ciliolabrum) – small myotis with medium-length ears. Straw-yellow dorsal fur (paler cream below) usually contrasts sharply with dark blackish (usually jet black) facial skin and ears. Difficult to separate from desert ecotype of M. californicus externally (this form occurs in the deserts, and also alongside ‘regular’ M. californicus in the Peninsular & Transverse Ranges and the SE slope of the Sierra), though the small tail tip protrudes slightly (less than half a cm) from the uropatagium (tail membrane) in WSFM. They also seem to have a broader, less hairy muzzle (hairless extent of snout is 1.5x the inter-nostril distance, compared to less than that in California Myotis), possibly longer & more pointed ears, and at some sites more contrast b/w dorsal fur and whitish ventral fur, but we’ll need more study to work out whether these are reliable field marks. Arid country California Myotis also often still show some pinkish or brown tones to their skin. Calls usually around 40-45 kHz are the best mark! Usually associated with rocky habitats in dry chaparral, oak woodland, arid grassland, sagebrush steppe, or desert; often roosts in caves or abandoned mines. I find foraging activity to peak later than other myotis in CA (which otherwise live up to their family’s name of “evening bats”), and I record most calls in e.g. Pinnacles 10PM-2AM.

LEFT: This Western Small-footed Myotis was photographed in the same (C CA) area as the first California Myotis shown above. It was identified acoustically as it flew in to night roost – note the pointed tip of the tragus that is just visible, which separates this species from Canyon Bat (Parastrellus). The lighter straw-yellow fur and very blackish ears & facial mask indicate this species over California Myotis, and possibly also the more contrasting fur (in this area) & broader, rather bare muzzle. RIGHT: This individual was photographed in the same area (S Sierra) as the bottom left California Myotis shown above, and also identified by acoustics as it flew briefly. But also note the paler fur, more blackish face & ears, and larger muzzle. I think it might also have a tiny protruding tail tip, which is often suggested as a field mark .

3. Long-eared Myotis (Myotis evotis) – large myotis with huge, curved blackish ears and brownish black facial skin. Pelage is typically two-toned, with the belly creamy white and dorsal fur medium brown to straw yellow. Very high bandwidth calls around 35 kHz are unmistakable. Only potentially confused with M. thysanodes, which has softer, lower frequency calls and shorter, wider, less curved ears. Widespread in rocky oak woodland, chaparral, and lower montane forest.

The impressive (and appropriately named) Long-eared Myotis is a large species with blackish mask & membranes and very long, oval-shaped ears that curve back at the tips.

4. Fringed Myotis (Myotis thysanodes) – large myotis with long ears; ears & facial skin pinkish-black. Pelage often strongly two-toned, with whitish belly contrastingly sharply with yellowish brown upper parts. I find the calls a bit softer than other myotis and regularly missed by the Echo Meter; usually around 25 kHz, typically below 30 kHz (thus separating this species from M. evotis). Often difficult to separate externally from M. evotis, but ears are generally shorter, more triangular in cross-section and less strongly curved. The “fringe” of stiff hairs along the edge of the uropatagium (tail membrane) is typically well-visible in close photos at the right angle (which are hard to take, honestly), or in-hand. Generally localized & uncommon in CA, but frequently found in some rocky chaparral sites in the coast range, old growth redwood forests of southern Humboldt county, and in the lower montane forests of the western Sierra Nevada, especially near Giant Sequoia groves.

TOP LEFT: Fringed Myotis is another impressive long-eared, large myotis. The ears are slightly shorter than those of Long-eared Myotis and are more triangular in shape (base much wider than tip), and the coloration is strongly two-toned. TOP RIGHT: The species is named for the trademark “fringe” of stiff hairs along the border of the tail membrane (uropatagium), obvious in-hand in this individual from NE Mexico. BOTTOM: This feature is hard to spot in situ, but I was lucky enough to manage this while watching this individual from the C Sierra (same as top left) – making a very likely ID a certain one!

5. Long-legged Myotis (Myotis volans) – medium-large myotis with short, rounded ears & pinkish brown to blackish facial skin (usually brownish). Coloration of dorsal fur variable, ranging from blackish in forested regions of northern CA, orangish brown, or golden brown with a paler belly. Calls around 40 kHz, but sonograms of this species are usually very difficult to separate from other 40 kHz myotis, as the distinguishing feature is very hard to record in the field. When seen roosting or in-hand, I find this bat most easily identified by a combination of its short, rounded ears and keeled calcar – a curved flap of skin that extends from the ankle. Fiona Reid says it has a short, very furry “bison-like” face, a field mark I like. Widespread in dense forest, usually at mid to high elevations but also common in pinyon-juniper woodlands E of the Sierra Nevada.

This sizable colony of Long-legged Myotis were found at a night roost in the southern Sierra. Note the rather short, very furry “bison face” (fur extending almost to the nostrils) and short, rounded ears typical of the species. Combined with the keeled calcar – a small, semicircular flap of skin extending from the inside of the ankle – these 3 features support a certain ID as M. volans.

In contrast to the golden-brown Long-legged Myotis (ssp. interior) of the E Sierra & much of the intermountain west, individuals on the Pacific slope of N CA belong to the blackish brown ssp. longicrus. This form also shows the typical short, rounded ears; short, furry face; and the keeled calcar.

6. Yuma Myotis (Myotis yumanensis) – small myotis with short to medium-length ears. Coloration very variable. In coastal northern CA dorsal pelage brownish black, with gray belly and blackish facial skin and ears; elsewhere in CA, usually dull grayish brown with pinkish gray facial skin & ears. In deserts of southern CA, fur blonde with pink facial skin, ears, and membranes. Calls around 50 kHz; usually associated with water (ponds, rivers, streams) and often forages by skimming the surface of stagnant or slowly-flowing bodies of water. Fur generally dull, though some individuals (seen in sites without M. lucifugus, so I’m sure I’m looking at yumanensis!) can show a somewhat glossy sheen to the dorsal fur. Calcar not keeled & feet fairly large. Widespread in CA at low elevations (most records < 8000 ft) near water, and one of the state’s most abundant bats. See M. volans (keeled calcar, larger), M. californicus (keeled calcar, more orangish fur contrasting with brown facial skin), M. lucifugus, M. occultus, and M. velifer.

LEFT: Yuma Myotis is a small species with medium-length ears; in most of CA, they have dull grayish brown fur and pinkish facial skin, ears & membranes, like this individual from the C Sierra foothills. RIGHT: Yuma Myotis of N coastal CA are similar in proportions, but have blackish pink (or even black) facial skin and dark blackish brown fur. Note the large foot characteristic of the species, which forages by trawling.

Yuma Myotis of the Lower Colorado River Valley, have pinkish gray facial skin & blonde fur.

7. Little Brown Myotis (Myotis lucifugus) – medium-large myotis with (usually) medium-length ears. Without calls, very difficult to separate externally from M. yumanensis in CA even by experienced field biologists. Dorsal pelage varies from blackish brown in coastal NW CA (ssp. alascensis) to grayish brown further inland in the Sierra Nevada & Cascades (ssp. carissima), and the venter is usually pale cream color. Skin on face and ears is blackish to pinkish gray. Dorsal fur is glossy/shiny; this is said by some sources to be a way to separate the species from M. yumanensis, but having seen somewhat glossy yumanensis I have my doubts. Calls around 40 kHz, and calls usually have longer duration than other 40 kHz Myotis (except the closely related occultus), but only in open uncluttered settings. In my experience, somewhat scarce in CA (especially compared to M. yumanensis), but grows more common above 8000 ft in the Sierra & locally common at some sites in NE CA and coastal NW CA (e.g. Lava Beds, Alturas).

This bat from NW CA was identified as a Little Brown Myotis as it lacked a keeled calcar (not visible in this photo), and I recorded 40kHz flight calls. Little Brown Myotis in NW CA (ssp. alascensis) mirror other myotis of the region in their dark (blackish brown here) pelage & facial skin. The species is practically impossible to separate externally from Yuma Myotis here, but seems to show on average longer ears and glossier fur, as this animal does. Little Brown Myotis of inland N CA (ssp. carissima) are often paler in color (varying from dark brown to pale grayish brown) & also very difficult to distinguish from local Yuma Myotis.

8. Arizona Myotis (Myotis occultus) – medium-large myotis with medium-length ears very similar to & split from M. lucifugus. In CA, restricted to the Lower Colorado River Valley where very rare. Dorsal pelage brownish & facial skin brown (often slightly darker than dorsal fur), compared to the (usually) blonde fur & pinkish facial skin of M. yumanensis in this area (per Allen Calvert). Calls around 40 kHz. Call parameters generally overlap with M. velifer but differ slightly in CA – per Allen Calvert (who has worked with both species in LCRV), occultus typically call at 38 kHz while velifer call at 40 kHz. M. velifer is also paler, with pinkish gray membranes.

Arizona Myotis is one of the rarest bats in CA, and as such this is a very special photo. Note the characteristic brown dorsal fur, brown facial skin & marked contrast between the brown back & pale belly – very distinct from the blonde fur & pinkish gray skin of the Yuma Myotis in this area. Compared to Yuma Myotis night-roosting nearby, this individual was also longer-eared and slightly larger; we also recorded 39 kHz flight calls while observing these bats. Allen Calvert – who surveyed bats for the LCR MSCP & caught most of the M. occultus known from CA in the last century – agreed that this is likely an Arizona Myotis.

9. Cave Myotis (Myotis velifer) – large myotis with medium-length ears found only along the Lower Colorado River in CA. I don’t know this species well in CA, as I’ve only seen it once emerging from a known roost. Fur color is dull grayish brown with pinkish gray membranes. Calls at 40 kHz. Externally, usually larger than M. yumanensis and said to be distinguished by a bare patch of skin between its shoulder blades, but I’ve heard this feature can be difficult to see in the field.

OTHER VESPERTILIONIDS: my catch-all for the remaining bats of CA.

1. Pallid Bat (Antrozous pallidus) – distinctive large, long-eared yellowish gray bat widespread in CA except at high elevations & in densely forested parts of N CA. Readily identified by its size and “pig nose”. Often night roosts under bridges or building eaves. California’s state bat as of 2023.

Pallid Bat is a distinctive large, yellowish gray bat with long, rounded ears and a blunt pig-like snout (left).

2. Big Brown Bat (Eptesicus fuscus) – medium-large, robust brownish bat with medium-length ears, long silky fur, and a large, bulbous/seemingly inflated nose. Pelage color ranges from warm brown (typical) to blonde (in the Lower Colorado River Valley), with the underside slightly paler. Calls usually 25-30 kHz. Membranes are usually blackish brown, but are often pinkish especially in more arid sites in southern CA.

Big Brown Bat is a robust, distinctive species readily identified by much larger size compared to myotis and their large, bulbous, snouts. Big Brown Bats have long, silky fur that is quite variable in coloration, as this colony from the Peninsular Range shows.

LEFT: Big Brown Bats are typically medium brown with blackish skin. RIGHT: Animals in the Lower Colorado River Valley often resemble Pallid Bats in coloration, with pale yellowish gray fur & pinkish skin!

3. Silver-haired Bat (Lasionycteris noctivagans) – medium-large blackish bat with short rounded ears and striking silver-tipped dorsal fur. Flight often slow and erratic, with some gliding. Not easy to identify by call – flat calls at or above 26 kHz are diagnostic. Migratory – in summer, one of the most common bats along forest streams in NW CA, and regular in the N Sierra Nevada; common during spring (Apr-mid May) and fall (Oct) migrations at oases in the N Mojave (e.g. Death Valley) & Great Basin. Roosts in foliage, underneath sheets of bark, and in crevices in snags (where very difficult to find).
4. Northern Hoary Bat (Lasiurus cinereus) – fairly large bat with short round ears and long wings. Coloration very distinctive, with a yellowish venter and grizzled gray dorsum. Flight style fairly fast and direct for a vespertilionid; often seen flying well before sunset, especially late fall-early spring. Readily identified when seen well in flight by robust silhouette and distinctive buffy coloration along the arms. Best identified acoustically from its long, flat calls at around 20 kHz, which have a shallow U-shape. Oscillogram widens at the middle, and is not “carrot shaped” (as in molossids, where the call’s power is highest at the beginning). Occasionally seen roosting in foliage of trees. Migratory; common in summer in NW CA & the Sierra Nevada, and in spring, fall, and less so winter in coastal parts of the state. Perhaps best seen in CA during spring or fall migration in coastal sites, often over meadows, golf courses, or city parks.
5. Desert Red Bat (Lasiurus frantzii) – beautiful medium-sized, round-bodied bat with short ears and frosted orange fur. Distinctively colored wings are black and orange/pink along the arms. Often observed flying in a rapid, erratic manner along the edges of riparian woodland, especially where they meet riverbeds or oxbow lakes. Calls approx. 40 kHz and are hockey stick to U-shaped; quite distinctive due to long duration and randomly varying characteristic frequency, duration, and bandwidth. Much of the population summers in the Central Valley, and some individuals remain here & in coastal CA year-round. Day-roosts in riparian belts among cottonwood and willow foliage.
6. Western Yellow Bat (Lasiurus xanthinus) – another robust, very distinctive medium-large bat with short ears and bright yellow fur. In CA almost always associated with fan palm (Washingtonia filifera) oases of the Colorado Desert; the bats roost in palm fronds and will often be seen & heard (listen for loud rustling in palm skirts) entering and exiting. Calls 30-35 kHz and show randomly varying characteristic frequency & duration.
7. Canyon Bat (Parastrellus hesperus) – tiny, short-eared bat with whitish yellow to straw yellow fur contrastingly sharply with black ears, facial skin, and membranes. Abundant in dry, rocky habitats across C & S CA including chaparral, oak woodland and arid grassland in the Coast Ranges; Colorado, Mojave & S Great Basin; and locally in N CA surrounding the Sacramento Valley. Largely absent from NW CA. Often seen flying beginning at sunset, especially near cliffs or over pools of water, with fluttering, moth-like flight; often curious and readily identified by coloration & flight style in decent light. Distinctive hockey-stick shaped calls at 45 kHz. Roosts (by day & night) in small rock crevices (where regularly found by herpers). Externally similar to Western Small-footed & California Myotis, but has short, blunt (“club-shaped”) tragus & a broad, black hairless snout.

Canyon Bat is a tiny bat that can be distinguished from California & Western Small-footed Myotis by its relatively larger muzzle & more naked facial region and short, blunt-tipped club-shaped tragus. Despite seeing loads in flight & a few on netting surveys, I actually don’t have photos of the species so thanks to Jon Hall (mammalwatching.com) for sharing this image taken during a survey in TX.

8. Townsend’s Big-eared Bat (Corynorhinus townsendii) – very small, delicate and tiny-bodied brown bat with huge curved ears. Typically seen roosting in caves or talus caves (e.g. those in Pinnacles NP), but may day or night-roost in buildings or under bridges. Widespread as scattered populations throughout the state, but fragile due to dependence on caves or large areas of rocky outcrops for roosting. Much smaller than M. evotis, with a tiny, shorter face with a lump on the nose. Can fold back its ears, making them appear shorter. Slow, fluttering flight.

LEFT: Townsend’s Big-eared Bat is a remarkable species with long ears nearly the length of its (tiny) body, small eyes & face, uniform brown fur, and a lump-like nose. RIGHT: Note the wart-like lumps on the noses of these Townsend’s Big-eared Bats and the folded ears (appearing much shorter) on some individuals.

9. Spotted Bat (Euderma maculatum) – spectacular, remarkable bat with huge pink ears & pink membranes; dorsal fur is black with a few large white spots, belly white. Calls are easily audible, very low frequency at ~10 kHz, and shorter in duration than Eumops (the only bat overlapping in frequency), thus sounding to my ears more like ascending clicks than the metallic chirps of molossids. Flight is fairly high but slow and erratic, at times resembling a huge moth. Day-roosts in cracks located on craggy cliffs in dry habitats and does not night roost, flying long distances all night to feed; day roosts are extremely difficult to find. Widespread but poorly-known in C, S & E CA; locally common in the granite country on the W slope of the C & S Sierra (e.g. Yosemite Valley), and also in the Great Basin (e.g. Owens Gorge & White Mountains). Regularly heard in appropriate habitat but usually does not fly low enough to be seen well.

The unmistakable Spotted Bat is one of CA’s most iconic mammals; netting this individual with researchers on a field survey in the Kaibab Plateau (AZ) was one of the highlights of my mammal watching “career.” Only a few of us took photos to reduce stress on the animals, so thanks to Jon Hall (mammalwatching.com) for sharing his image of the bat we saw together in 2018!