Silver-haired Bat (Lasionycteris noctivagans)

This article originally appeared in the State of Washington Bat Conservation Plan. Further information is available from the Washington Department of Fish and Wildlife.

Image copyright Merlin D. Tuttle, Bat Conservation International,
Image copyright Merlin D. Tuttle, Bat Conservation International,

Description. The silver-haired bat is a medium- sized bat (Table 5) with black or dark brown hairs tipped in silver (Kunz 1982b, Van Zyll de Jong 1985, Nagorsen and Brigham 1993). The wings and tail membrane are black. Ears are short and round with a short, blunt-tipped tragus. The dorsal surface of the tail membrane is partially furred and the calcar lacks a keel.

Taxonomy. No subspecies are recognized (Simmons 2005).

Distribution. Silver-haired bats range broadly across North America from southeastern Alaska across the southern half of Canada south through most of the contiguous U.S. and into northeastern Mexico (Kunz 1982b; Appendix A). The species is present throughout Washington (WDFW WSDM database).

Silver-haired bats are migratory across much of their range, with males and females appearing to occupy separate summer ranges over broad regions (Wilson and Ruff 1999, Cryan 2003). Males are usually predominant in western North America, whereas females occur mainly in midwestern and eastern regions, although there are deviations to this pattern,  including  in  the  Pacific  Northwest. Male:female ratios have not been widely documented in Washington. Campbell (1993) reported  an  even  sex  ratio  among individuals (n = 30) captured from May to September in Stevens and Pend Oreille counties, whereas Baker and Lacki (2004) caught only males (n = 81) from May to August in Kittitas and Yakima counties. Females comprise 37% of the museum specimens (n = 118) collected year-round from across the state, including 30% during winter (n = 27), 47% in spring-summer (n = 38), and 34% in fall (n = 53). These data appear to refute Perkins and Cross’ (1991) suggestion that the two sexes are geographically separated in Washington, but increasingly occur together as fall migration progresses in late August and September.

Population status. Population size and trends are unknown throughout the species’ range (WBWG 2005), but it usually occurs at low densities (NatureServe 2009). Surveys in Washington indicate that silver-haired bats are common in drier forests along the east slope of the Cascade Mountains in Kittitas and Yakima counties (Baker and Lacki 2004) and in the Selkirk Mountains (Campbell 1993). The species was regularly detected during acoustic and/or capture surveys in the northern Cascades (Christophersen and Kuntz 2003), southern Cascades (Thomas 1988, Erickson 1993), at Mount Rainier National Park (Petterson 2001), at Joint Base Lewis-McChord (Falxa 2008a), at Woodard Bay in Thurston County (Falxa 2007b), in the San Juan Islands (Dalquest 1940), in Spokane County (H. Ferguson, pers. comm.), and at several sites in Lincoln County (BLM, unpubl. data). In riparian areas of the Columbia Basin in eastern Washington, these bats were recorded uncommonly at Hanford (Gitzen et al. 2002) and the Yakima Training Center (Christy et al. 1995), and were uncommon or rare at Moses Coulee, where some detected individuals may have been migrants (Fleckenstein 2000, Rosier and Rosenberg 2006). However, a second survey at Hanford found it to be widespread and the second-most commonly recorded species (Lindsey et al. 2012). West et al. (1984, 2004) captured only single individuals during surveys in the southwestern Cascades and on the Olympic Peninsula, respectively.

Habitat. Silver-haired bats typically reside in for- ests and riparian zones (Kunz 1982b, Nagorsen and Brigham 1993, Johnson and Cassidy 1997, Verts and Carraway 1998). Older, more structurally di- verse forests generally appear preferable to young- er, intensively managed forests due to differences in roost availability and canopy structure suitable for foraging (Perkins and Cross 1988, Thomas 1988, Betts 1998a, Jung et al. 1999). In Washington, this species occurs in forests and suburban/developed areas, and has been described as largely absent from shrub-steppe except during migration (Per- kins and Cross 1988, Johnson and Cassidy 1997). However, Gitzen et al. (2002) and Lindsey et al. (2012) documented the calls of silver-haired bats at Hanford throughout summer, and Fleckenstein (2000) captured a few individuals at Moses Coulee in summer. This suggests the species is a summer resident in some areas of shrub-steppe; clumps of trees found in riparian areas or on farmsteds in this habitat may be sufficient to support resident popu- lations. Elevations from sea level to at least 1,830 m are used (Nagorsen and Brigham 1993, Petterson 2001, Christophersen and Kuntz 2003).

Roosts and roosting behavior. Silver-haired bats roost most commonly in snags and live trees, including ponderosa pine, Douglas-fir, lodgepole pine, western white pine, western larch, western redcedar, grand fir, aspen, and black cottonwoods (Populus balsamifera ssp. trichocarpa; Nagorsen and Brigham 1993, Betts 1996, 1998a, Campbell et al. 1996, Mattson et al. 1996, Vonhof 1996, Crampton and Barclay 1998, Vonhof and Gwilliam 2007). Buildings (especially exteriors), bat houses, and wood piles are also regularly occupied in Washington (G. Falxa, pers. comm.). Maternity colonies usually contain 5-25 females and have rarely reached 70 individuals (Rainey and Pierson 1994, Mattson et al. 1996, Vonhof and Barclay 1996, Betts 1998a, Vonhof and Gwilliam 2007). Maternity groups roost mainly in cavities in large snags in various stages of decay, especially those protruding above the surrounding canopy, being farther from other tall trees, and having less vegetative matter immediately above and below the roost site (Table 3; Betts 1996, 1998a, Campbell et al. 1996, Mattson et al. 1996, Vonhof and Barclay 1996, Vonhof and Gwilliam 2007). These attributes may promote absorption of solar radiation and retention of heat that provide thermoregulatory benefits to reproductive females. Reproductive females change their roosts regularly (e.g., once a day to once every 18 days or longer; Betts 1996, 1998b, Vonhof and Barclay 1996).

Males and non-reproductive females roost solitarily away from nurseries (Humphrey 1975, Mattson et al. 1996). Their day roosts occur in large trees in intermediate stages of decay under loose bark, in cracks or crevices, and in cavities (Mattson et al. 1996). Solitary individuals may switch roosts daily or less frequently (Campbell et al. 1996, Mattson et al. 1996).

Information about night roosts is lacking, but the species rarely if ever occurs at sites (e.g., bridges, buildings) commonly used by other bat species (Perlmeter 1996, Pierson et al. 1996, Adam and Hayes 2000). During migration, silver-haired bats roost mainly in trees (Barclay et al. 1988, McGuire et al. 2012). Most migrating individuals roost alone, although single bats may occur in different parts of the same tree. Other roost structures used during migration include buildings, lumber piles, fence posts, utility poles, and mines (Barbour and Davis 1969, Nagorsen and Brigham 1993, McGuire et al. 2012; BLM, unpubl. data). Migrating bats may remain torpid for several days during cool temperatures (Barclay et al. 1988).

Silver-haired bats occupy a variety of winter roost sites, including trees, buildings, abandoned mines, and more rarely in rock crevices and caves (Kunz 1982b, Maser 1998). In western Washington, buildings, trees, and bat houses are occupied at this time of year (G. Falxa, pers. comm.). Caves do not appear to be widely used as hibernacula in the state (Perkins et al. 1990). In British Columbia and Washington, winter roost trees include large western redcedar, large Douglas-fir trees and snags, and decadent big-leaf maples, with crevices and sites beneath loose bark being occupied (Cowan 1933, Nagorsen and Brigham 1993; G. Falxa, pers. comm.). Hibernation roosts require stable microclimates (Humphrey 1975). Ambient temperatures of –0.5 to –2ºC (28.4–31.1°F) have been reported in old mines used as hibernacula (Nagorsen and Brigham 1993). During winter, silver-haired bats hibernate and/or use daily torpor depending on the severity of weather conditions. Hibernation and winter daily torpor may be interspersed with bouts of foraging, especially in western Washington (Falxa 2007a). Silver-haired bats winter alone or in small groups that can contain both sexes (Humphrey 1975, Kunz 1982b).

Reproduction. Males have enlarged testes from July to September in the Pacific Northwest (Maser 1998, Baker and Lacki 2004). Mating likely occurs during autumn migration and winter (Kunz 1982b, Nagorsen and Brigham 1993, Cryan et al. 2012). Sperm is probably stored in the uterus during winter followed by ovulation and fertilization in late April and early May (Kunz 1982b). Pregnancy lasts 50-60 days, with births occurring in late June or early July (van Zyll de Jong 1985, Nagorsen and Brigham 1993). Females give birth to one or two young, with two being most common  (Barbour and Davis 1969, Kunz 1982b, Parsons et al. 1986, Nagorsen and Brigham 1993). Lactation lasts about 36 days (Kunz 1982b). Young are able to fly by three weeks of age (Nagorsen and Brigham 1993). In Washington, volant young have been detected beginning in early August (Campbell 1993, Baker and Lacki 2004). Most juvenile males and females reach sexual maturity in their first autumn (Kunz 1982b, Cryan et al. 2012).

Food habits and foraging. In the Pacific North- west, this species forages mainly on moths, flies, beetles, leafhoppers, true bugs, neuropterans, and caddisflies (Whitaker et al. 1977, 1981b, Nagorsen and Brigham 1993, Kellner and Harestad 2005, Lacki et al. 2007b, Ober and Hayes 2008a). Moths and flies are important prey in some coastal wet for- ests (Whitaker et al. 1977), with evidence of mod- erate dietary specialization on moths in inland dry forests (Whitaker et al. 1981b, Lacki et al. 2007b).

Foraging typically occurs in and over forests and riparian zones; over openings, streams, and ponds; and along forest margins (Whitaker et al. 1977, 1981a, Kunz 1982b, Thomas 1988, Thomas and West 1991, Johnson and Cassidy 1997). In western Washington, peaks in nocturnal activity occur for several hours after sunset and again before sunrise (G. Falxa, pers. comm.). Foraging may be reduced on summer nights with cool air temperatures (<8°C, <46°F) (Nagorsen and Brigham 1993), but is known to extend through winter in Washington (Falxa 2007a). Adults generally forage singly, although pairs and groups of 3-4 bats are also observed (Barbour and Davis 1969). During migration, silver-haired bats feed along intact riparian areas in arid rangelands (Whitaker et al. 1981b). Because of their short broad wings, low- to mid-frequency echolocation calls, and slow agile flight, they are able to detect and capture small insects at close range (Barclay 1985, 1986, Nagorsen and Brigham 1993).

Seasonal movements. Most northern populations migrate to the more southern parts of the species’ range to overwinter (Izor 1979, Wilson and Ruff 1999, Cryan 2003). However, Washington’s population is comprised of both year-round residents and migratory individuals. Museum records and detections of foraging and roosting animals suggest that large numbers of silver-haired bats occur year- round in western Washington (Johnson 1953, Falxa 2007a; G. Falxa, pers. comm.; G. Green, pers. comm.), whereas smaller numbers are present in eastern Washington (E. Rowan, pers. comm.; N. Williams, pers. comm.). It appears that significant numbers of individuals also migrate through the state, as indicated by mortality records from wind energy facilities and other data (Perkins and Cross 1991, Johnson and Erickson 2011). Data from wind farms show that spring migration in Washington occurs from about late April to late May and that fall migration lasts from about mid-August  to late October (e.g., Kronner et al. 2008; numerous other unpublished reports). This species appears to migrate singly or, less often, in small groups (Barbour and Davis 1969, Barclay et al. 1988). In eastern North America, fall-migrating silver-haired bats move in waves, make brief stopovers of one or two days before continuing onward, and generally forage on non-travel nights (McGuire et al. 2012). Individuals fly about 250-300 km per night while migrating (McGuire et al. 2012).

Threats. During migration, silver-haired bats are one of the species most commonly killed at wind farms, including in Washington (Arnett et al. 2008, Johnson and Erickson 2011). Given the species’ use of snags for roosting, particularly large snags for maternity sites, forestry practices that greatly reduce existing snags and curtail development of large snags may adversely affect local populations. Loss of temporary roosts along migration routes is a potential threat, as is loss of foraging habitat in riparian areas and reduction of prey due to application of pesticides (WBWG 2005).

Conservation measures. Pre-construction surveys of proposed wind energy facilities should be made to establish the timing and location of potential conflicts so that mitigation measures can be used to reduce mortality to this species. At existing wind farms, surveys are needed to document mortali- ties and measures are needed to reduce mortalities. Forestry practices should maintain an abundance of large snags to provide a diversity of potential roost sites so that the different seasonal thermoregula- tory needs of males and females are met (Betts 1998a, Vonhof and Gwilliam 2007). To be suitable as maternal roosts, snags should be large in diam- eter (>60 cm dbh depending on site and species), tall, in the early stages of decay with retention of most of the stem, and positioned in a way that in- creases their conspicuousness and exposure to so- lar radiation, such as protruding above the canopy and being isolated from other tall trees (Betts 1996, 1998a, Campbell et al. 1996). In inland dry forests, Campbell et al. (1996) recommended that snags be situated in canopy  gaps or open areas >100 m upslope from riparian areas. Providing small groups of suitable snags may increase use of a site by silver-haired bats because of the availability of alternate roosts in close proximity (Campbell et al. 1996). Documentation of the temporal and spatial distribution of this species throughout Washington, including important migratory pathways, will help inform conservation measures and the appropriate time to apply them.

About the Author: 
We thank Greg Falxa, Lori Salzer, Ella Rowan, Travis Nelson, Mike Ritter, Nancy Williams, Jon Lucas, Neal Hedges, John Fleckenstein, Laura Ellison, Robert Fischer, Patti Happe, Ellen Myers, Mason Reid, Joan St. Hilaire, Kent Woodruff, John Bassett, Steve West, Greg Green, Derek Stinson, Keith Aubry, and Jason Lowe for providing useful information on various topics during the preparation of the plan. Final peer review comments were kindly provided by Joe Buchanan, Greg Falxa, Ella Rowan, Harriet Allen, Howard Ferguson, Nancy Williams, John Fleckenstein, Ted Weller, Tom Rodhouse, Lori Salzer, John Bassett, Katie Gillies, Greg Green, Lynn Hellbrecht, Doug Kuehn, Cori Lausen, Cole Lindsey, Jon Lucas, Michael MacDonald, Mike Ritter, Travis Nelson, Michelle Tirhi, and Mitch Wainwright. Comments on earlier drafts were received from Joe Buchanan, Paula Call, Greg Falxa, John Fleckenstein, Michael MacDonald, Patti Happe, Sally Nickelson, Pat Ormsbee, Ella Rowan, Jim Schaberl, Dave Waldien, Ted Weller, Steve West, and Ron Wohrle. Lori Salzer and John Talmadge assisted with creating the bat species range maps for Washington. Kelly Cassidy, Charles R. Conner Museum, Washington State University; Gary Shugart, Slater Museum of Natural History, University of Puget Sound; and Philip Myers, Museum of Zoology, University of Michigan, provided records of bat specimens held at their institutions. Bat Conservation International ( provided (with permission) the color photographs and North American range maps of each bat species used in the species accounts and Appendix A, respectively. Staff at the Washington State Library gave invaluable assistance in obtaining published literature.