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PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES 
Series 4, Volume 65, Supplement I 
distance of 1.1 kilometers (range 0.1 to 2.7) between consecutive roosts (Evelyn et al., 2004). Trees 
used as roosts had larger diameters (mean of 1.15 m, greater than found in most studies of other 
species of tree-cavity roosting bats), and were taller than randomly selected and neighboring trees, 
with diameter appearing to be the key variable associated with roost tree selection; at the site level 
roost trees were closer to water and located in areas with higher forest cover than randomly select¬ 
ed comparison points (Evelyn et al., 2004). 
Two maternity colonies of Yuma myotis numbering 60 and over 500 bats have been reported 
occupying basal hollows of redwood trees in northern California (Gellman and Zielinski, 1996). 
Cliff swallow (Petrochelidon pyrrhonota ) nests were occupied by this species during August after 
the breeding season in the Verde Valley of central Arizona (Vaughan, 1980). 
Warm Season Roosts in Caves and Mines: A maternity colony of “several thousand” Yuma 
myotis was reported in a cave near Pyramid Lake, Nevada in 1924 (Hall, 1946:135), and a mater¬ 
nity colony of unspecified size was observed in a shallow cave near Del Rio, Texas in 1903 (Bai¬ 
ley, 1905). They were captured at mouths of two caves at elevations of 2,770 and 3,014 meters 
(three bats at one cave, 14 at the other) in Colorado during summer, but type of use was unspeci¬ 
fied (Siemers, 2002). Three abandoned mines were the most northerly maternity roosts (two colony 
sizes of 500-750 and one of 50-60) known in Idaho by Betts (1997). These roosts had higher and 
more constant relative humidity than unused mines, more constant temperatures, and were also less 
likely to be disturbed by people. Howell (1920a) reported a colony of about 600 females in May 
1918 segregated at 30- and 60-meter depths in the Senator Mine on the lower Colorado River in 
Imperial County, California, with those bats closest to the entrance roosting singly and torpid, and 
those deeper in the mine active and in clusters. An abandoned mine used as a maternity roost was 
reported in the Moapa Valley of Nevada (Williams et al., 2006). A deep abandoned mine shaft has 
been used as a migratory stopover roost by these bats in New Mexico (Altenbach et al., 2000). 
Use of abandoned mines by maternity colonies of Yuma myotis also has been documented 
recently in southwestern Arizona, southeastern California, and southern Nevada (Henry, 2002; 
Williams et al., 2006; Brown, 2013). One abandoned mine on Imperial National Wildlife Refuge 
in southwestern Arizona has housed a maternity colony ranging to over 3,000 adults since annual 
monitoring took place from 2001 to 2013 (Brown, 2013), but was reported to harbor about 9,000 
in 1994 (Castner et al., 1995). Another abandoned mine on the California portion of the refuge held 
a maternity colony of about 2,000 Yuma myotis (Brown, 2013). Other colonies in abandoned mines 
in this region include about 1,500 individuals at one mine in the lower Colorado region of south¬ 
eastern California, and the recent discovery of an abandoned mine housing a maternity colony of 
up to 5,500 bats on Bureau of Land Management property in San Bernardino County, California 
(Brown, 2013). 
Warm Season Roosts in Buildings and Bridges: Buildings were the sites of most early 
records of Yuma myotis roosts. Dalquest (1947b) described roosts in multiple buildings in Califor¬ 
nia and noted that all were near water, near trees, were dimly lit, and most provided dark crevices 
where bats roosted. Early records of roosts of this species included a maternity colony from aban¬ 
doned buildings at old Fort Tejon, Kern County, California, in which 61 bats, all females or young, 
were taken in July 1904 (Grinnell, 1918). This site was known to house a large colony of these bats 
as early as 1891, but it was apparently unoccupied by this species by 1945 (Dalquest, 1947b). They 
were found roosting in a warehouse at San Simeon in San Luis Obispo County on the California 
coast, and a lone male was reported from a crevice in an abandoned house in Santa Clara County, 
California (Dalquest, 1947a). 
Use of buildings by maternity colonies of Yuma myotis is widespread. The attic and belfry of 
a church in Wadsworth, Nevada housed a maternity colony of about 5,000 bats (Dalquest, 1947b) 
