Seasonal and geographic trends in acoustic detection of tree-roosting bats

Migratory routes, timing, and behavior are some of the least studied facets of bat biology, and possibly play roles in bat mortality rates observed at commercial wind energy facilities. We used acoustic detectors to record seasonal activity of silver-haired (Lasionycteris noctivagans), hoary (Lasiurus cinereus), and eastern red (Lasiurus borealis) bats above the forest canopy at one existing and 13 proposed wind energy facilities in seven eastern U.S. states between April and November 2007 and 2008. We correlated species detection rates between surveys located within three predetermined geographic regions, and correlated species detection rates from two survey locations with mortality reported from a nearby commercial wind facility. We identified 2,603 L. noctivagans, 1,908 L. cinereus, and 6,802 L. borealis calls during 6,153 detector-nights. We found a greater number of significant correlations between sites for silver-haired and hoary bat detection rates than in eastern red bat detection rates. Each species exhibited unique seasonal trends in detections among geographic regions. Previously reported mortality rates of L. noctivagans and L. cinereus from a wind energy facility were positively correlated with detection rates of those species at one of our survey locations within 50 km (r = 0.65, P < 0.001 and r = 0.28, P < 0.01, respectively; in both cases d.f. = 94) and with another location within 100 km (r = 0.44, P < 0.001 and r = 0.28, P < 0.01, respectively; in both cases d.f. = 81). These data indicate that seasonal detection rates of all three species under study reflect their different migratory patterns that may be useful in predicting the timing of mortality events at wind energy facilities.