What Happens to Bats in the Pacific Northwest Winter?

U.S. Fish and Wildlife Service
5 min readNov 12, 2020

During the summer evenings, lucky observers can witness a bat flying acrobatically as it forages on pesky insects. But as leaves start to change on the trees and days grow shorter with more rain in the Pacific Northwest, fewer and fewer bats are seen. So where do these nighttime critters go in the winter? That is a question biologists in the Pacific Northwest are trying to answer. There are at least 16 bat species in the Pacific Northwest, all of which have their own strategies for surviving the winter months.

closeup of bat face
Hoary bat. Photo by Daniel Neal/Creative Commons Attribution 2.0 Generic license.

Some bat species will migrate long distances in the fall to warmer climates where insects are more plentiful. In the Pacific Northwest, the hoary bat (Lasiurus cinereus) is known for its long-distance travels; in one study this species was found to fly more than 1,000 km during its migration. Not much is known on the migratory behavior of these bats such as where they end up spending their winters, but we do know that their migration path crosses into areas where wind energy is being developed. Across the country, hoary bats have been found dead at wind energy facilities. Luckily, many researchers are working with the wind energy industry to find ways to eliminate or reduce the number of deaths of these migratory bats.

Flying long distances to get through the winter isn’t for every bat. Some species remain active regardless of the season. For example, we are learning some species that live in coastal and lowland Puget Sound areas in the Pacific Northwest are active year-round. The California myotis (Myotis californicus) and silver-haired bat (Lasionycteris noctivagans) are two of the bat species that have been found in the winter months in these areas. It’s believed the bats can go into torpor (a shortened form of hibernation) in inclement weather to conserve energy, but in periods of good weather, enough insects are available to sustain these populations through the winter.

bats hanging upside down on ceiling of cave
Townsend’s big-eared bats hibernate together in cave in Washington. Photo by Abby Tobin/WDFW.

For those bats that do not fly long distances or remain active, hibernation through the winter is key. Where bats hibernate is still somewhat a mystery. Unlike many of the species in eastern North America, most species do not form large colonies (hibernacula) in the winter, rather they are more dispersed on the landscape likely using a variety of roosts for hibernation. Some species have been found hibernating in caves or abandoned mines, such as the Townsend’s big-eared bat (Corynorhinus townsendii), which will gather together with hundreds of others during hibernation.

2 photos of single bat upside down in cave
Big brown bat and little brown bat hibernating in cave in Washington. Photo by: Shawn Thomas/WDFW (left) and Abby Tobin/WDFW (right).

Other species have also been found hibernating in similar spots but in fewer numbers, such as the silver-haired bat, western long-eared myotis (Myotis evotis), Yuma myotis (Myotis yumanensis), big brown bat (Eptesicus fucus) and little brown bat (Myotis lucifugus). It’s also thought that many bats will select other places to roost that offer protection and stable yet cool temperatures for hibernation such as tiny crevices in cliffs, talus slopes or even in root wads.

2 pics show monitoring posts
Acoustic monitoring posts in Washington. Photos by Abby Tobin/WDFW.

Biologists in Washington State are using a variety of survey techniques to locate winter roosts. Acoustic monitoring and radio telemetry are among some of the methods being used. Acoustic monitoring utilizes a specialized microphone to detect the high frequency sounds emitted by bats as they echolocate. To echolocate, bats send out sound waves from the mouth or nose. When the sound waves hit an object they produce echoes. The echo bounces off the object and returns to the bats’ ears.

bat on a gloved hand with antenna coming out of back
Little brown bat with radio transmitter being tracked to locate hibernacula in Washington. Photo by Abby Tobin/WDFW.

This data provide insight to which species are where and when, helping to solve the mystery of where bats hibernate. In addition, small radio transmitters can be attached to bats allowing biologists to follow them to the places where they roost. This sounds simple enough, but catching these cryptic fast flying animals can be tricky, as is tracking them on foot or from a vehicle or plane.

bat with WNS
Bat in hibernation with white-nose syndrome. Photo by USFWS.

It’s important for biologists to understand how bats spend their winters because there is a bat disease called white-nose syndrome (WNS) that is affecting bats during hibernation. This disease is caused by a fungus and does not affect humans or other animals, but can be deadly to bats. It is estimated that more than 6 million bats have died since first being discovered in eastern North America in 2006. Unfortunately, in 2016 WNS was discovered in Washington and since then has spread to six counties in the state (wdfw.wa.gov/bats). There is even more urgency now to figure out more about bat winter roosting ecology so biologists can better protect these important animals.

You can help this winter by reporting any bat observations to your local wildlife agency (wdfw.wa.gov/bats). In addition, please do not touch or disturb any bats during their wintry rest. We can all play a part in learning about and helping native wildlife and ecosystems. Bat Week was last month, but this site has great tips on what you can do for bats.

By Abby Tobin, Acting Columbia Pacific Northwest White-Nose Syndrome Coordinator, Washington Ecological Services Office, USFWS



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