: The western pipistrelle is the smallest of Colorado's bats. It is easily recognized by its slow, erratic, butterfly-like flight. It has short, black ears, grayish brown pelage and a distinctive black mask. It is one of the most attractive bats in North America. It weighs less than 4 g, little more than a penny. Other measurements are: wingspan, 190-215 mm; total length, 68-75 mm; and forearm, 29-31 mm.
Distribution: This species is found throughout the desert southwest. It ranges as far north as Washington state and east to Oklahoma. In Colorado, it is found at lower elevations on the Western Slope and in the extreme southeast.
Habitat and Habits: This is a bat of the desert. It lives in arid canyons or dry shrub lands, near water. The western pipistrelle rarely roosts in human-made structures, nor does it seem to use mines or caves much except as hibernacula. It does roost in dense vegetation and beneath rocky slabs. Although the western pipistrelle doesn't use the same roost day after day, it roosts in the same general area. Day and night roosts are different. This bat is non-migratory and sedentary. The only hibernaculum discovered to date in Colorado was in a gold mine at 9,500 feet in the La Plata Mountains above Mancos. Predators are probably snakes, birds of prey and other bats.
Breeding: One or two young are born in June each year. The western pipistrelle was confirmed to breed in Colorado when an infant bat was discovered on the floor of an abandoned building at Rio Blanco Reservoir, and a lactating female and a juvenile were observed at the Colorado National Monument. These bats may form small nursery colonies of 20-50 females and young. The young are very small and exhibit the black mask at an early age. Males are segregated from females during much of the summer. Copulation occurs in the hibernaculum, followed by ovulation in spring.
Food: Western pipistrelles eat small moths, beetles, mosquitoes and other flies. The western pipistrelle emerges very early to forage, sometimes before sundown, and again in early morning. The animals forage near canyon walls and among scattered boulders and shrubs. They remain active throughout the year. Foraging seems to be limited by winds above 10 mph.
Remarks: The size of this bat can only be appreciated at close range. In the air, it appears fragile. The slightest breeze blows it off course or causes it to stall.