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PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, Supplement I 
far fewer were taken elsewhere at Big Bend, leading the investigator (Easterla, 1973) to believe 
that a maternity colony was located in nearby cliffs (all big-free-tailed bats taken there were adult 
females or volant juveniles). Overall at Big Bend National Park, Easterla (1973) captured 411 indi¬ 
viduals among 4,807 bats of 18 species, ranking fifth in relative abundance when the single site 
with 391 captures of this species is included. They ranked seventh in relative abundance among 17 
species (85 of 1,978 bats captured) during a subsequent survey at Big Bend National Park during 
1996-1998 that emphasized lowland habitats, with most captures over open water in river flood- 
plain habitat (Higginbotham and Ammerman, 2002). Big free-tailed bats ranked least abundant 
among 14 species (one out of 542 individuals) captured in mist nets that sampled at 108 locations 
over water in northern Chihuahuan desert habitats at Big Bend Ranch State Park in the Trans-Pecos 
region of Texas; a single bat was captured over water in a sheer canyon with steep, rocky cliffs 
(Yancey, 1997). 
Foraging and Dietary Analysis.— The high aspect ratios and wing loading of big free¬ 
tailed bats are analogous to those of fast flying aerial insectivorous birds with foraging habits like 
swifts and swallows (Vaughan, 1966). Like other molossid bats, they often fly at higher altitudes 
above ground where fewer obstructions exist to interfere with their less-maneuverable flight. Five 
were successfully radio tracked to determine the extent of nightly movements (including foraging) 
in rugged terrain at House Rock Valley in northern Arizona (Corbett et al., 2008). Flight speeds of 
at least 61 kilometers per hour were measured, with bats ranging as far as 36 kilometers from 
roosts. An activity area was estimated for one of the females tracked for six nights: she covered 
29,590 hectares while foraging, a much larger area than reported for other species of bats but per¬ 
haps typical for big free-tailed bats in the region (Corbett et al., 2008). In Mexico City, they have 
been documented to forage widely over urban areas based on echolocation activity, but they 
favored large parks, forests, and illuminated areas; these favored habitats had greater insect abun¬ 
dance than other areas, and it was noted that the design of the echolocation calls of this species 
allows detection of prey at longer distances in open areas such as the airspace over these urban fea¬ 
tures (Avila-Flores and Fenton, 2005). 
Morphological specializations of the head and limited information on food habits would sug¬ 
gest that this species feeds primarily on moths (Freeman, 1979, 1981). Macrolepidopteran moths 
(probably sphinx moths) were the only group detected in the gastrointestinal tract of the single 
specimen examined by Ross (1964, 1967). In the most extensive study of stomach contents, East¬ 
erla and Whitaker (1972) examined 49 individuals from Big Bend National Park in Texas and also 
found large moths to be by far the most important dietary component (at 86% proportional vol¬ 
ume), but they also noted the presence of more terrestrial insects (crickets and katydids, families 
Gryllidae and Tettigoniidae) at up to 50% volume in 14 stomachs; two individuals contained large¬ 
ly ants (Formicidae). Freeman (1981) found moth parts to be the dominant prey items that could 
be identified in fecal samples from four individuals. Debelica et al. (2006) examined fecal pellets 
from 40 individuals captured at Big Bend National Park during the months of May-September in 
2001 and 2002. They reported similar diets to those documented by Easterla and Whitaker (1972) 
from the same region over 30 years earlier, with lepidopterans the dominant item (about 87%) and 
other orders constituting less than 5% each by volume. Sparks and Valdez (2003) found a more var¬ 
ied diet in analysis of fecal pellets from below a roost in northern New Mexico, where although 
lepidopterans were the most frequently occurring group, the most important food items by propor¬ 
tional volumes were homopterans (cicadellid leafhoppers) and hymenopterans (ichneumonid 
wasps), followed by lepidopterans, hemipterans, and dipterans. It is possible that some of the 
smaller, weaker fliers among insect groups reported in dietary analyses are swept aloft by rising air 
currents. 
