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Migration and Hibernacula (Winter Roosts) of Big Brown Bats in Colorado

Movements, distribution, and roosting requirements of most species of temperate zone bats in autumn are poorly understood. We conducted the 1st radiotelemetry study of autumn migrations and pre-hibernation roost selection of bats in western North America. Big brown bats (Eptesicus fuscus, n = 55) in the Poudre River watershed, Colorado, moved from low elevation summer ranges to high elevation locations in autumn, where they roosted in rock crevices during the period leading up to winter hibernation. We presumed these bats hibernated alone or in small numbers in any one rock crevice. Lack of evidence for hibernating aggregations suggests limited opportunity for rabies transmission in winter, when the literature shows that the rabies virus is dormant as well. However, local movements over distances up to 87.4 km from Fort Collins suggests a potential for the virus to occasionally be transmitted over a wider area, particularly if mating takes place near wintering areas.

We characterized rock crevices used as roosts in autumn at these higher elevations at microhabitat and landscape scales. We used logistic regression combined with an information theoretic approach to determine which variables were most important in roost selection. At the microhabitat scale autumn roosts were higher to the ground above and below the exit point and were in deeper crevices that had more constant temperatures than randomly selected crevices. At the landscape scale, aspect of the hillside was important, with autumn roosts typically facing north-northwest. Autumn roosts fell into 2 categories: those used for a few days (transient roosts) and those used for ≥7 days and presumed to be hibernacula.

Temperature regimes in the presumed hibernacula appear to provide optimal conditions for use of winter torpor, whereas transient roosts may offer passive re-warming and energy savings for bats still active in early autumn.

Elevational segregation of sexes was also documented in our region with a preponderance of females found at lower elevations and males at higher elevations in summer. Sex ratios at higher elevations became even in autumn. Use of short elevational migrations and selection of hibernation sites in rock crevices may be a common overwintering strategy of insectivorous bats of western North America.