Roosts and food supplies ameliorate the impact of a bad summer on reproduction by the bat, Myotis lucifugus LeConte (Chiroptera : Vespertilionidae)

At the Chautauqua institution in southwestern New York State, the period be tween May and September in 1991 differed in rainfall and temperature from t he same period in 1992. 1992 was significantly cooler and wetter than 1991, reflecting the global impact of the 1991 eruption of Mt Pinatubo. Some bui ldings at the Institution are used as nursery roosts by thousands of Myotis lucifugus (little brown bats) and in one roost, maximum temperatures were significantly lower in 1992 than in 1991. There were no significant differe nces between these two summers in the diet of M. lucifugus, which mainly co mprised aquatic insects that did not differ significantly in availability. There were significant differences in the numbers of bats using some roosts between years. with more bats in larger colonies and fewer in smaller colo nies in 1992. This suggests greater use of clustering by roosting bats, whi ch was supported by direct observation. There were no significant differenc es between years in the timing of reproduction (incidence of pregnant, lact ating and post-lactating females or the appearance of volant subadults). We conclude that roosts and food supply appear to insulate M. lucifugus from the effects of cool, wet weather.