DRAFT
March 2012
MAMMALS Hoary Bat (Lasiurus cinereus)
Hoary Bat
(Lasiurus cinereus)
Legal Status
State: None
Federal: None
Critical Habitat: N/A
Recovery Planning: N/A
Notes: N/A
Taxonomy
The hoary bat is in the family Vespertilionidae and is designated by its current scientific name Lasiurus cinereus, which was first used in 1864 (Shump and Shump 1982; Wilson and Reeder 2005), although Hall (1981) placed it in the genus Nycteris. There are three recognized subspecies of hoary bat, L. c. cinereus, L. c. semotus, and L. c. villosissimus (Wilson and Reeder 2005). L. c. cinereus is the North American subspecies treated in this species profile. L. c. semotus is restricted to Hawaii, and L. c. villosissimus is restricted to South America (Shump and Shump 1982).
Although the single species Lasiurus cinereus appears to be clearly distinct from other species in the genus, there is some uncertainty in the designation of the number of species recognized for the genus Lasiurus based on morphologic, cytogenetic, and molecular analyses (Baker et al. 1988; Marchesin et al. 2008; Morales and Bickham 1995). Following Morales and Bickham (1995), the genus Lasiurus in North America is divided into three groups based on fur color—hoary bats, red bats, and yellow bats—but this trichotomy is still unresolved, with most of the variability related to the red and yellow bat groups. There is some evidence that the hoary and red bats are more closely associated and that the yellow bats may represent a separate genus, but this issue is unresolved (Morales and Bickham 1995). There is no evidence indicating that the hoary bat represents more than one species (Morales and Bickham 1995), and there is no current information to indicate any pending taxonomic changes for the hoary bat. The species’ physical characteristics are described in detail in Shump and Shump (1982).
Distribution
General
The hoary bat is the most widespread of all North American bat species (Shump and Shump 1982). It occurs throughout the United States, except for the southern tip of Florida, throughout central and eastern of Canada, and in Central Mexico (Figure SP-M4). It is common in the prairie states and the northwestern United States, and is less common in the eastern United States and northern Rockies (Shump and Shump 1982).
The hoary bat winters in Southern California and the southeastern United States (Shump and Shump 1982). However, in California during the fall and winter, males and females appear to have elevational separation, with males occurring at higher elevations in the foothills and mountains, and females at lower elevations in the lowlands and coastal valleys; however, these patterns are still not well understood (Cryan 2003; Shump and Shump 1982; Vaughn and Krutzsch 1954).
Distribution and Occurrences within the Plan Area
Historical
The California Natural Diversity Database (CNDDB) contains 10 historical (i.e., pre-1990) records for hoary bat that date between 1905 and 1989 (CDFG 2011a). These occurrences are scattered throughout the Plan Area, with three occurrences in the Little San Bernardino Mountains; two occurrences along the lower Colorado River; and single occurrences in the Victorville area, the Clark and Granite mountains in eastern San Bernardino County, Death Valley in Inyo County, and the Chocolate Mountains in Imperial County. There are two additional CNDDB records within 5 miles of the Plan Area boundary in the Owens Valley and along the northern flank of the San Gabriel Mountains. Most the historical records are imprecise or simply refer to the species being collected or observed along a riparian area or stream. Of the 12 historical records, 10 are for the March to May period, indicating the spring migration period; 1 is for July; and 1 is for October. Most of the records are for females.
Recent
There are no recent (i.e., since 1990) CNDDB records for the hoary bat in the Plan Area, although there is one record from March 1998 within 5 miles of the Plan Area boundary in the Peninsular Range in eastern San Diego County (CDFG 2011a). It is assumed that the lack of recent records reflects survey effort rather than a decline in the abundance of the species in the Plan Area because there is no clear reason to suggest that this species’ occurrence in the Plan Area should be substantially reduced, although some areas of historic occurrences (e.g., the Victorville area and the lower Colorado River) have experienced increased human development in recent decades. Loss or degradation of riparian woodland habitat in the Plan Area is also a potential reason for the lack of recent records. Also, the few historic records span a period of about 85 years compared to the 22 years for “recent” records.
Natural History
Habitat Requirements
Hoary bats typically roost in tree foliage and sometimes cavities, such as woodpecker holes, 3 to 5 meters (9.8 to 16.4 feet) above the ground (Constantine 1966; Shump and Shump 1982). A torpid hoary bat was reported in a squirrel’s nest in Georgia (Neill 1952). In Iowa, Constantine (1966) observed that foliage provided dense shade and cover above the roost. However, even on a particular tree, they may select roost sites that provide an appropriate microclimate and open flyways. Willis and Brigham (2005), for example, characterized roost sites in mature white spruce (Picea glauca) in Saskatchewan, Canada. Sites were located on the southeast side of trees at the same height as the surrounding forest. There was less canopy cover facing out from the tree in the direction of the roost site, thus providing an open flyway. Although temperatures were not different from other sides of the tree, these southeast-facing sites provided the roosts with protection from the prevailing westerly winds, which resulted in a significant energy savings (Willis and Brigham 2005). Hoary bats may also select roosts that are away from human disturbance. Constantine (1966), for example, only found roosts in trees bounding crops distant from human populations.
Most foraging probably occurs in open areas within forest, woodland riparian, and wetland habitats, but foraging habitat in the arid southwest includes juniper scrub, riparian forest, and desert scrub (Shump and Shump 1982). Barclay (1985) observed that hoary bats primarily foraged in the lee of a narrow forested ridge in Manitoba, Canada, where conditions were less windy. In northern New Mexico, migrating females foraged below the canopy along streams (Valdez and Cryan 2009).
Potential habitat associations for hoary bat in the Plan Area are shown in Table 1.
Table 1. Habitat Associations for Hoary Bat
Land Cover Type / Habitat Designation / Habitat Parameters / Supporting Information /Forest, woodland, riparian / Roosting and foraging / Forest, woodland, riparian / Shump and Shump 1982; Valdez and Cryan 2009
Desert wash, desert scrub, Pinyon-Juniper scrub, wetland / Foraging / Desert wash, desert scrub, pinyon-juniper scrub, and wetland within 6.2 miles of roosting habitat / Barclay 1989; Shump and Shump 1982
Foraging Requirements
Hoary bats feed on Lepidoptera (moths), but also on Coleoptera (beetles), Diptera (flies), Orthoptera (grasshoppers), Blattodea (termites), Odonata (dragonflies), and Hymenoptera (wasps) (Black 1972, 1974; Shump and Shump 1982). They emerge from day roosts 30 minutes to 2 hours after sundown, but also in the late afternoon in winter (Barclay 1989; Shump and Shump 1982). They may be active after midnight in the southwest (Shump and Shump 1982). Foraging behavior by females is related to stages of reproduction. At a study site in Manitoba, Canada, females emerged to forage earlier in the evening and foraged for longer periods of time as lactation progressed through the reproductive season (Barclay 1989). Also, females with two pups foraged longer than females with one pup and females foraged in different habitats pre-fledgling and post-fledgling (Barclay 1989). Barclay (1989) observed reproductive females foraging in one to five bouts per night totaling 3 to 6 hours, separated by intervals spent with young. Foraging activity, including time spent foraging, early in the lactation period may be constrained because young are vulnerable to ambient conditions in exposed roosts, thus requiring frequent returns to the roost (Barclay 1989).
Valdez and Cryan (2009) characterized foraging habitat use during spring migration in New Mexico. Hoary bats primarily fed on moths below the stream canopy along streams, but the focus on moths appeared to decline in late spring, possibly due to seasonal abundance or differential prey selection. Valdez and Cryan (2009) suggest that spring migration through New Mexico is timed to coincide with prey availability.
Reproduction
As noted previously under Distribution, in California, males and females appear to be spatially separated and only come together for mating, primarily in autumn, although mating may also occur at common wintering sites (Shump and Shump 1982) (Table 2). Females are not known to give birth and rear young in California (Shump and Shump 1982). Prior to giving birth, females migrate to northern and northeastern portions of North America, where they give birth in mid-May to July (Shump and Shump 1982). At least in some portions of their summer range (e.g., southern Canada), females can enter prolonged torpor during spring storms and delay birth until conditions improve (Willis et al. 2006). Litter size is usually two pups (range of one to four), with pups born relatively undeveloped (i.e., only partially furred, eyes closed, etc.) (Bogan 1972). They can fly by day 33 and exhibit well developed flight (e.g., purposeful swoops) by day 44 (Bogan 1972).
Reproductive females use solitary roosts in tree foliage and sometimes cavities, such as woodpecker holes, at heights of 3 to 5 meters (9.8 to 16.4 feet) above the ground (Shump and Shump 1982), and they may select roosts sites based on microclimate factors (Willis and Brigham 2005 [see Habitat Requirements]). Koehler and Barclay (2000) suggest that postnatal growth rate and breeding biology of solitary tree-roosting species such as hoary bat may be different from colonial species that roost in caves, mines, buildings, and other protective structures due to tree roost habitats and greater exposure to ambient conditions during the reproductive season. A litter of two pups is larger than most colonial species that only have one pup per season. Postnatal growth rates are correlated with ambient temperatures and rainfall. As discussed in Foraging Requirements, females’ foraging activity patterns and time away from the roost are related to the developmental stage of the young (Barclay 1989). Also, slow postnatal growth may be a life history trait of migrant bat species such as hoary bat that can forage year round (Koehler and Barclay 2000).
Table 2. Key Seasonal Periods for Hoary Bat
/ Jan / Feb / March / April / May / June / July / Aug / Sep / Oct / Nov / Dec /Breeding / ? / ? / x / x / x / x
Birth/rearing of young / x / x / x / x
Migration / x / x / x / x / x
Wintering / x / x / x / x / x / x
______
Sources: Cryan 2003; Shump and Shump 1982.
Spatial Activity
The hoary bat is a migratory species. Its lower wingload makes it one of the fastest fliers of North American bats, and it may fly in a wide temperature range of 0 to 22 degrees Celsius (32 to 72 degrees Fahrenheit) (Shump and Shump 1982). Its migratory speed can reach more than 21 kilometers per hour (13 miles per hour).
Although migration by hoary bats is well documented and it is known to winter in California, its overwintering sites in Mexico are not well understood (Cryan 2003). Migration patterns differ between fall and spring, with fall migration groups being larger and more organized than spring migrations (Cryan 2003; Shump and Shump 1982), which may account for the greater incidence of wind turbine strikes in late summer and fall (see Threats and Environmental Stressors). Northward migrations from wintering grounds in California and Mexico occur in the spring, and there is little evidence that hoary bats migrate north from South America (Cryan 2003). Female migration precedes male migration by about 1 month (Valdez and Cryan 2009). During the summer, males are primarily found in the western and females in the eastern portions of North America (Cryan 2003; Findley and Jones 1964). Hoary bats may occur in California year-round, but there is little evidence that females give birth and rear young in California, as noted previously under Reproduction. Females appear to migrate farther than males, possibly to find suitable sites to raise young (Cryan 2003). Females likely need more resources because they have greater physiological requirements to support reproduction. Also, the thermal environment of the maternity roost may affect the growth rate of young (Cryan 2003); therefore, finding suitable roosting sites likely is important. In late summer, female hoary bats embark on coastward migrations, moving along the North Pacific Coast and Farallon Islands in the west; breeding may occur at this time (Cryan 2003).
There are relatively few data for foraging distances from roost sites, and actual distances are likely to vary locally in relation to prey availability. Barclay (1989) recorded straight-line flight distances up to 20 kilometers (12.4 miles) by reproductive female hoary bats in Manitoba, Canada. However, flight distances to suitable foraging habitat for this species may not be a constraint because they are fast fliers, although energetic considerations may be a factor.