Food Habits of the Hoary Bat (Lasiurus Cinereus) During Spring Migration Through New Mexico (Report‪)‬

Hoary bats (Lasiurus cinereus) are among the most wide-ranging mammals in North America, and their seasonal movements illustrate the unique migration strategies of bats (Findley and Jones, 1964; Cryan, 2003). Behaviors and distributions of male and female bats often are decoupled during spring and summer (Fleming and Eby, 2003). For example, female hoary bats leave wintering grounds in California and Mexico earlier than males during spring, and tend to migrate farther into northern and eastern regions of the continent than males (Findley and Jones, 1964; Cryan, 2003). In New Mexico, female hoary bats captured during spring usually are pregnant (Bogan, 1972; Druecker, 1972) and less likely than males to use energy-saving torpor when faced with cold temperatures (Cryan and Wolf, 2003). This energetically demanding combination of pregnancy and defense of normothermic body temperatures likely imposes additional challenges to the already risky behavior of long-distance migration. How do hoary bats, especially pregnant females, minimize the risk of starvation or energy deficits that impede gestation while migrating through areas subject to inclement weather? Do hoary bats actively feed during spring migration to offset such energy demands? If so, do they exploit specific sources of food or use a more generalized feeding strategy? We captured hoary bats as they migrated through New Mexico during spring 2002 and collected guano to try and better characterize the food resources they exploit during this critical period. We also report the first anecdotal observations on flight behavior and possible group travel of hoary bats during spring migration. MATERIALS AND METHODS--We captured L. cinereus at three proximate sites in the Sandia Mountains east of Albuquerque, Bernalillo Co., New Mexico, during 8 May-25 June 2002. These sites included San Pedro Wash (35[degrees]12'N, 106[degrees]18'W), upper Tijeras Canyon (35[degrees]4'N, 106[degrees]23'W), and lower Tijeras Canyon (35[degrees]4'N, 106[degrees]25'W); sites were netted for 16, 5, and 9 nights, respectively. All capture sites were within 18 km of each other, 1,830-1,970 m in elevation, and situated in riparian areas associated with small streams (1-3 m wide) running through pinon juniper (Pinus-Juniperus) woodlands and mixed-conifer forests. Riparian vegetation along stream banks was dominated by narrow-leaf cottonwood (Populus angustifolia), Siberian elm (Ulmus pulmia), and willow (Salix). We captured bats in mist nets set over pools in flowing or intermittent streams. Nets were monitored continuously, and we recorded the time that each bat was captured, as well as noted when multiple bats were captured together ([+ or -] 10 s). Bats were promptly removed from nets, and we determined sex, age, and reproductive condition using standard methods (Anthony, 1988; Racey, 1988). Bats were placed in clean, individual cloth bags and held for [greater than or equal to]15 min for collection of guano. Mist nets typically were opened 2-4 h/night of sampling. Tissue biopsies were taken from plagiopatagia of most (86%) bats as part of another study, thus providing marks that allowed us to determine if recaptures were common. All bats were released at the initial site of capture.