Northern California Redwood Forests Provide Important Seasonal Habitat for Migrant Bats

Bats are known to roost in redwood forests year-round, but their activities outside the summer season are poorly understood. To improve understanding of the use of redwoods by resident and migrant bats, we conducted 74 mist net surveys between February 2008 and October 2010. Captures were dominated by Yuma myotis (M. yumanensis) in the summer and silverhaired bats (Lasionycteris noctivagans) in the winter. During November-February, silverhaired bats, accounted for 78 percent of 23 captures and male:female sex ratio was (9:9). By contrast, during June-August, silver-haired bats accounted for 13.8 percent of 269 captures and sex ratios were highly male skewed (34:3). In combination with other regional information, this indicates that female silver-haired bats migrate to redwood forests. To infer summer locations of bats captured in redwood forests, we analyzed stable isotopes of hydrogen, carbon, nitrogen, and sulfur in their fur. Despite spatial segregation between male and female silver-haired bats during presumed molt period, we did not find differences between the sexes in range of isotope values in their fur. Nor were their values different from Yuma myotis. Our findings highlight some of the challenges in using stable istotope analysis to infer migratory pathways in bats.