Redwood Forests May Be Crucial for Silver-Haired Bats

Evaluating the Importance of Redwood Forests as Wintering and Mating Habitat for a Continental Migrant: the Silver-Haired Bat

Silver-haired bats mate in redwood forests. Photo by Theodore J. Weller
Silver-haired bats mate in redwood forests. Photo by Theodore J. Weller

When Ted Weller heard about bats flying over a frozen stream in Canada, he was intrigued. “I always considered winter a dead time for bats,” he said. “I figured they were mostly hibernating.”

A US Forest Service ecologist, Weller decided to check out his own backyard: the redwood forests of Northwest California. He not only found bat activity in winter, but also important clues about the bats’ migrations. When Weller had surveyed a common species called the silver-haired bat in summer, he’d found almost all males. In the winter, however, he began to catch females right away. So he asked Save the Redwoods League to fund research to figure out what was going on.

Dozens of mist-net surveys later, Weller found that while the ratio of male to female silver-haired bats in the summer was 34 to 3, in the winter it was 15 to 12—closer to equal. The bats were arousing from torpor (temporary hibernation) not to feed, but probably to find mates.

And where had the females been all summer? Weller was unable to pinpoint the location. But he knew from others’ research that silver-hairs are sexually segregated in the summer, with the females migrating perhaps hundreds of miles to raise their pups in places that are warmer and richer with insects. During the fall and winter, females come back to the redwoods.

Eventually, Weller hopes to use stable isotopes (species of atoms of a chemical element) in bat fur to find out more about the species’ migration routes. But simply knowing where males and females congregate is a big step forward. “Mating habitat is critical to the conservation of this species,” Weller said. “Hence redwood forests of Northwestern California could prove a vital area for maintenance of silver-haired bat populations at regional or, perhaps, continental scales.”

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