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At the Chautauqua Institution in southwestern New York State, the period between May and September in 1991 differed in rainfall and temperature from the same period in 1992. 1992 was significantly cooler and wetter than 1991, reflecting the global impact of the 1991 eruption of Mt Pinatubo. Some buildings 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 differences between these two summers in the diet of M. lucifugus, which mainly comprised 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 colonies in 1992. This suggests greater use of clustering by roosting bats, which was supported by direct …