A small, drab-gray or smoke-gray bat with distinct, black, leathery facial mask and black membranes; tragus short, blunt, and slightly curved; underparts pale smoke-gray. Dental formula: I 2/3, C 1/1, Pm 2/2, M 3/3 X 2 = 34. External measurements average: (males), total length, 66 mm; tail, 27 mm; foot, 5 mm; forearm, 28 mm; (females), 73-30-5-28. Weight, 3-6 g.
Habits. This bat is associated chiefly with rocky situations along watercourses. Its daytime retreat is in the cracks and crevices of canyon walls or cliffs, under loose rocks, or in caves.
These are among the most diurnal of bats, beginning their foraging flights very early in the evening and often remaining active throughout the early morning hours. Pipistrelles are slow bats and may be distinguished on the wing by their slow, fluttery flight which is restricted to small foraging circuits. Occasionally, individual bats have been observed on the wing during mid-day, during which time they water to alleviate stress caused by the arid environment they inhabit.
Western pipistrelles forage from 2 to 15 m above ground on small, swarming insects and consume about 20% of their body weight in insects per feeding. Specific prey items include caddisflies, stoneflies, moths, small beetles, leaf and stilt bugs, leafhoppers, flies, mosquitoes, ants, and wasps. Stomach contents of individual bats often contain only a single species of insect, or if more than one species is present the remains are clumped together within the stomach, suggesting that they take advantage of swarming insects and feed intensively within such swarms.
The young, numbering one or two (usually two), are born in June and July after a gestation period of approximately 40 days. Maternity colonies may be established in buildings or rock crevices. The newborn bats weigh slightly less than 1 g at birth but grow rapidly. By August they can fly and are difficult to distinguish from adults.