Pipistrellus hesperus (Western Pipistrelle Bat)

Weight: 3-6 grams

Wingspan: 19-22 centimeters


Southern Washington to southern Mexico.


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Ecology and Behavior:

The western pipistrelle bat is one of the smallest bats in the United States. Primarily a desert species, it inhabits a variety of habitats from rocky canyons, cliffs, and outcroppings to creosotebush flats. Day roosts usually are in rock crevices, but may be beneath rocks, in burrows, in mines, and in buildings. It tends to roost singly or in small groups; a maternity colony of 12 individuals is the largest known group of this species. In winter, it has been found hibernating in mines, caves, and rock crevices. Among the most diurnal of bats, it often begins foraging flights before sunset and may remain active well after dawn. However, except for lactating females that may be active throughout the night, early evening activity usually ceases within 1-2 hours after sunset. The flight is fluttery and is among the slowest and weakest of all our bats. A slight breeze can bring these bats to a standstill, and a stronger wind may cause them to seek shelter.

Food Habits:

Forages 2-25 meters (7-82 feet) above ground on swarming insects, and consumes about 20% of its body weight per feeding. Prey items include caddisflies, stoneflies, moths, small beetles, leaf and stilt bugs, leafhoppers, flies, mosquitos, ants, and wasps.

Reproduction and Longevity:

Twins are born in June or July, after a gestation of about 40 days. Newborn bats weigh less than 1 gram, but they grow quickly. Juveniles begin to fly at about 1 month of age.

Status of Populations:

Common in the desert Southwest.

Text, in its original form, provided by T. L. Best, M. J. Harvey, and J. S. Altenbach. Printed spectrographs provided by M. J. O'Farrell. Distribution maps, call descriptions, and AnaMusic sound clips produced by W. L. Gannon. Accounts assembled by T. C. Sanchez-Brown.