Flight Activity of Bats 83 



the final model. No pattern was observed between significant and 

 nonsignificant environmental variables using coefficients of variation 

 (Table 2), indicating no bias because of differences in the amount of 

 variability in these sets of variables. 



DISCUSSION 



We found a direct relationship between flight activity of P. t. 

 virginianus and ambient relative humidity, with bats exhibiting 

 reduced activity on nights with lower percent relative humidity. Using 

 mist-net captures as a measure of activity, Lacki (1984) observed a 

 similar pattern for Myotis lucifugus and suggested reduction in flight 

 activity as a behavioral mechanism for avoiding water loss on 

 nights when ambient conditions created large vapor pressure deficits. 



Bats experience water loss in flight and can lose water because of 

 roost conditions (Carpenter 1969). Substantial water loss accompanies 

 digestion in M. lucifugus (Bassett and Wiebers 1980) with water 

 balance in female M. lucifugus (Kurta et al. 1989) and female Eptesicus 

 fuscus (Kurta et al. 1990) influenced by reproductive condition. Because 

 water loss by bats is dependent on ambient temperature and water 

 vapor pressure under laboratory conditions (Bassett 1980) and water 

 loss increases with higher levels of flight activity (Studier 1970), we 

 suggest that our data support the existence of a behavioral response 

 by bats for avoiding extremes in vapor pressure deficits during flight. 



Factors such as prey activity and availability may also contribute 

 to the observed activity patterns, but were not investigated in our study. 

 Data comparing the abundance of insect prey with ambient relative 

 humidity are sparse; however, in one study no association was found 

 between relative humidity and activity of moths (Mizutani 1984). 

 Moths are the predominant item in the diet of P. t. virginianus (Sample 

 and Whitmore 1993). 



The inverse relationships we observed between activity of P. t. 

 virginianus and both moon phase and wind speed are consistent with 

 other findings reported in the literature (O'Farrell et al. 1967, Erkert 

 1974, Fenton et al. 1977). Whether avoidance of moonlight by P. t. 

 virginianus was because of predators or availability of insect prey or 

 both is unclear. Several species of owls were common in the study 

 area, and abundance of insect prey has been shown to be negatively 

 related to moon phase (Anthony et al. 1981). Observations at a maternity 

 colony of a related subspecies, the Ozark big-eared bat (P. t. ingens), 

 found no patterns between flight activity and indicies of moon 

 brightness (Clark 1991). 



