Influence of Environmental Conditions on Flight 



Activity of Plecotus townsendii virginianus 



(Chiroptera: Vespertilionidae) 



Michael D. Adam 1 , Michael J. Lacki, and Laura G. SIioemaker 



Department of Forestry, University of Kentucky, 



Lexington, Kentucky 40546 



ABSTRACT — Flight activity of the Virginia big-eared bat (Plecotus 

 townsendii virginianus) was measured in relation to eight environ- 

 mental variables during 1990 and 1991 in Lee County, Kentucky. 

 Activity, measured as the mean nightly detection frequency of 

 bats fitted with transmitters, was positively related to percent 

 relative humidity and negatively related to moon phase and wind 

 speed. Multiple regression analysis showed relative humidity to 

 have the strongest association with flight activity of all the environmental 

 variables tested. An explanation for this pattern was that bats 

 reduced their foraging activity on nights of low relative humidity 

 to avoid excess water loss because of extremes in vapor pressure 

 deficits during flight. Other explanations for the observed activity 

 patterns may exist, but they were not investigated in our study. 



A wide range of abiotic environmental variables affect flight 

 activity of bats, including sunlight, moonlight, temperature, wind speed, 

 and precipitation. Sunlight inhibits flight activity and serves to 

 synchronize circadian periodicity (Erkert et al. 1980). Moonlight 

 reduces flight activity (Erkert 1974) and is known to induce shifts in 

 foraging patterns (Fenton et al. 1977). Flight activity of bats increases 

 with temperature, with shorter activity periods on cooler nights (Anthony 

 et al. 1981) and extended bouts of activity on warmer nights (O'Farrell 

 et al. 1967). Sufficiently strong winds suppress flight activity (O'Farrell 

 et al. 1967), but the influence of slower air speeds, if any, is unknown. 



Responses of bats to precipitation is not consistent among, or 

 even within, species. Heavy rainfall reduced flight activity of 

 Pipistrellus pipistrellus (Stebbings 1968), but did not do so in another 

 study (Swift 1980). The timing of rainfall events is important 

 (Felton et al. 1977), with rain at dusk known to delay nightly emergence 

 in Nycticeius humeralis (Watkins 1972). 



In contrast, effects of relative humidity on flight activity of 

 bats have been suggested (Watkins 1972, Lacki 1984) but quantitative 

 data are lacking. Studies have demonstrated the importance of water 

 balance to bats under both laboratory (Bassett 1980, Bassett and 



1 Present address: Coastal Oregon Productivity Enhancement, Hatfield Marine 

 Science Center, 2030 South Marine Science Drive, Newport, Oregon 97365-5296. 



Brimleyana 21:77-85, December 1994 77 



