Winter Foraging Behavior of Silver-Haired and California Myotis Bats in Western Washington

The wintering strategies of many species of bats in the Pacific Northwest are poorly understood (Barbour and Davis 1969; Kunz 1982). Although Silver-haired Bats (Lasionycteris noctivagans) and California Myotis (Myotis californicus) have been observed wintering in Washington State and British Columbia (Nagorsen and others 1993), the specifics of their wintering behaviors and distributions in the region are unclear. Silver-haired Bats are presumed to migrate south to warmer regions during winter (Izor 1979), although limited evidence suggests this species may not be as migratory in the Pacific Northwest as elsewhere (Schowalter and others 1978; Kunz 1982; Cryan 2003). The few observations of winter roosts used by Silver-haired Bats in the Pacific Northwest mostly involved solitary bats found in trees (Cowan 1933; Nagorsen and others 1993). California Myotis are presumed to hibernate through the winter and generally remain dormant in caves, mines, buildings or tree cavities within several hundred kilometers of their summer habitat (Christy and West 1993; WDFW 2004). During the winters of 1982 through 1989, Perkins and others (1990) surveyed 650 caves and mines and 50 buildings in Oregon and Washington for use by bats. In Washington they found 2 species of hibernating bats (Myotis evotis and M. volans) but did not encounter Silver-haired Bats or California Myotis. O’Farrell and Bradley (1970) noted several reports of winter foraging of insectivorous bats in the southwestern United States, but information on winter foraging by bats in the Pacific Northwest has not been published. Most earlier reports of wintering bats in Washington involved observations of hibernating and roosting individuals (Senger and others 1974; Izor 1979; Perkins and others 1990). A California Myotis found flying during winter inside a gymnasium at the University of British Columbia in Vancouver had insects in its stomach (Nagorsen and Brigham 1993). Additional information on the winter feeding habits of Silver- haired Bats and California Myotis in the Pacific Northwest is not available. The collection records of the Burke Museum of Natural History at the University of Washington and Slater Museum of Natural History and the University of Puget Sound include information on 22 Silver- haired Bats and 18 California Myotis collected between November and February in Washington; one Silver-haired Bat was noted as being active when collected. Johnson (1953) described recovering the latter specimen at Point  Defiance Zoo after it flew from bushes and was found to have sparse fat reserves and an empty stomach.


Falxa, G.A. 2007. Winter Foraging Behavior of Silver-Haired and California Myotis Bats in Western Washington. Northwestern Naturalist 88(2): 98-100. doi: 10.1898/1051-1733(2007)88[98:WFOSAC]2.1.CO;2

Download PDF