O’SHEA, CRYAN & BOGAN: UNITED STATES BAT SPECIES OF CONCERN 
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Roosting Habits. — Winter Roosts: Winter roosts of big free-tailed bats in the U.S. are 
unknown. Records of occurrence during winter are rare compared to summer. Although they are 
capable of torpor (LaVal, 1973), some sources suggest that these bats migrate long distances sea¬ 
sonally, whereas others note that it is not clearly known whether they migrate or hibernate locally 
(for example, Higginbotham and Ammerman, 2002; Ammerman et al., 2012a). Poche (1979) noted 
substantial fat deposition in September in southwestern Utah, with multiple captures during late 
May through mid-September but no captures during other months. 
Warm Season Roosts: In the United States, the big free-tailed bat is primarily a dweller of 
crevices in cliff faces, although use of tree cavities and buildings is known in other countries to the 
south (Milner et ah, 1990). The very few roosting sites discovered thus far in the U.S. have been 
in rock crevices, particularly in cliffs of steep-walled canyons (e.g., Borell, 1939; Poche, 1979; 
Bogan et ah, 1998; Navo and Gore, 2001). Borell (1939) reported the first colony known from the 
United States, discovered in a canyon in the Chisos Mountains of southwestern Texas (elevation 
1,890 meters) based on loud daytime vocalizations emanating from a high rock crevice (Borell, 
1939). About 130 were housed in the small horizontal crevice (about six meters long and 15 cen¬ 
timeters wide) located about 13 meters above the talus slope of a sheer cliff wall (Borell, 1939). A 
presumed nursery colony also was reported from a vertical rock crevice formed by an exfoliating 
rock slab about 13 meters above a canyon floor in Chihuahua, Mexico near Big Bend National 
Park, Texas, where additional colonies in crevices in canyon walls were reported (Easterla, 1972, 
1973). Many of the mist-netting records also have been in the vicinity of habitat with cliffs within 
canyons (Easterla, 1973; LaVal, 1973; Mollhagen and Bogan, 2016), as have the few acoustic sur¬ 
vey records in California (Pierson and Rainey, 1998c). 
Poche (1979) reported a colony of about 150 of these bats in a crevice in a cliff in southwest¬ 
ern Utah during summer. A maternity colony in a rock crevice along the Los Pinos River in north¬ 
western New Mexico and a maternity colony “under slabs of lava on a perpendicular lava cliff’ in 
central New Mexico discovered by hearing loud daytime vocalizations were reported by Findley et 
al. (1975:70). In the Jemez Mountains of north-central New Mexico, five lactating females were 
radio tracked to colonies in five south and east facing roosts in rock crevices, nine to 35 meters 
above the bases of high canyon walls at elevations ranging 1,921 to 2,311 meters (Bogan et al., 
1998). Emergence counts in the Jemez Mountains study averaged 100 bats per roost (range six to 
over 220 bats); radio-tagged bats ranged far from roosts, which were located 11-30 kilometers 
from the point of capture (Bogan et al., 1998). 
Seven radio-tracked adult females led to the discovery of three roosts in the rugged habitats of 
far northern Arizona (Corbett et al., 2008). Roosts were in large, vertical crevices in tall (30 to 152 
meters) cliffs of south or southeast facing canyon walls at distances ranging from 1.9 to 23.2 kilo¬ 
meters from points of capture over stock ponds (roosts are described in greater detail by Corbett et 
al., 2008); four bats exited one of these roosts, but estimates of colony size were not possible at 
other roosts (Corbett et al., 2008). In southwestern Colorado, three adjacent roosts in crevices in 
canyon walls above the Dolores River were discovered in summer of 1998 after investigators heard 
loud daytime vocalizations; 25 individuals were seen emerging from the only crevice where con¬ 
ditions permitted observation (Navo and Gore, 2001). Neubaum (2017) located three roosts in 
crevices in large cliffs in western Colorado, two found opportunistically and one by radio tracking 
a non-reproductive female. Counts at emergence at the three roosts ranged from at least thirteen to 
64 bats. A maternity colony is also known from a canyon in southeastern Colorado (Navo and Gore, 
2001). Pierson and Rainey (1998c) noted the likely existence of a roost in a rock crevice high on a 
cliff face in San Diego County, California. 
There are records of two specimens of big free-tailed bats from buildings in San Diego, Cali- 
