Temperate-zone bats feed almost exclusively on night-flying insects, and must either migrate or hibernate in winter when their insect prey is unavailable. Hibernating bats emerge from caves or mines in spring, but the cues that they use to determine when spring has arrived are not well understood. Bats hibernating deep within a cave or mine do not receive outside light and temperatures may show little seasonal fluctuation. We examined the phenology of bat emergence in spring from the Neda Mine Bat Hibernaculum, an abandoned iron mine in central Wisconsin that houses approximately 150,000 bats each winter, primarily Little Brown Bats (Myotis lucifugus). We instrumented the mine with infra-red photo beam-break detectors that allow us to record the number of bats entering and leaving the mine on a 5 minute interval continuously, and estimated when bats emerged from hibernation each spring from 2001-2007. We related bat activity in spring to environmental variables including temperatures (recorded in and outside of the mine), airflow, and barometric pressure. We looked for changes in temperatures and bat phenology over time to assess the potential effects of global warming, and correlated bat activity with environmental variables to assess what cues might be driving bat emergence.

Results/Conclusions

We found some evidence that the interior of the mine has been warming over time (1996-2007). Temperature increases within the mine have not yet affected bat phenology: there was no evidence that bats are emerging earlier, although we have a shorter run of data for bat activity (2001-2007). The dates of bat emergence from the mine were very consistent: the range from the earliest to the latest date of 50% emergence from the mine was 7 days over the 8 years of the study. Bat phenology was much less variable than dates of bud break of native plants recorded at a nearby site over the same time period, which showed ranges of 8-21 days between the earliest and latest date of 50% bud break, depending on the species. Changes in airflow, subtle changes in temperature within the mine, and changes in barometric pressure are all possible cues that may signal to the bats that spring has arrived.