Population Turnover in Wintering Bats in Indiana

Abstract

The turnover in winter cave bat populations has not been adequately studied, although bat banders (Beer, 1955; Rysgaard, 1942; Griffin, 1940; Guthrie, 1933) are aware that hibernating concentrations do not remain constant throughout any given season. The extent to which the animals move in and out of their hibernating quarters poses an interesting problem. Such shifting is important in evaluating winter populations, banding returns, and perhaps sex ratios (one sex may tend to shift about more than the other). If it can be demonstrated that wintering populations fluctuate from day to day, or week to week, some of the seemingly low rates of band returns may be explained. It is conceivable that the wintering population, though remaining relatively constant with regard to total numbers, may actually be composed of an ever-changing group of individuals. Unless the bats are banded, such population turnovers go undetected. As Beer (1955) has pointed out, bats are often retaken two or more years after banding, but not each successive year. This seems to indicate that they do not utilize the same wintering quarters year after year. It could also mean that, if the premise of constant turnover is true, some banded individuals are simply missed unless the cave is checked frequently throughout the winter season. Bats may utilize two or more suitable hibernacula, shifting from one to the other as natural movement, to facilitate mating, or in response to fluctuating changes in temperature. The big brown bat (Eptesicus fuscus), which is known to be quite a hardy species, seems well fitted to winter movements of these types.