Creature Feature: the Hoary Bat

A hoary bat hangs from a branch. Photo: Tom Benson, Flickr CC.

The hoary bat (Lasiurus cinereus) is one of North America’s largest bats, boasting a solid 5.9 inches in length. Named for its “hoary” fur—meaning silvery or grayish-white—the bat’s body is encased in cinnamon brown fur tinged in frosty silver. Its tiny face is surrounded by a furry golden halo, making these bats even more recognizable and adorable. The hoary bat is widespread throughout the U.S., found in 49 of 50 states.

Out of the thirteen species of bats that live in Humboldt County, the hoary bat is particularly unique due to the fact that it migrates and hibernates, when most bats do one or the other. This has befuddled scientists, but one potential possibility for this behavior relates to their roosting habits. Hoary bats are a tree roosting species, meaning they sleep and live outside in trees rather than caves, like many other bats, and they are one of the only species of bats that hibernate in the open. This could explain why they evolved for hibernation but choose to migrate in large numbers from the eastern states to northwest California in autumn to roost in the warm, moist, and sheltered redwoods.

Locally, Humboldt Redwoods State Park appears to be an important migratory hot spot for the hoary bat, with a seasonal concentration of mating bats not seen elsewhere on the planet. So many hoary bats, in fact, that Humboldt Redwoods may be drawing bats from all over western North America!

Hoary bats are important insect predators, with a diet consisting primarily of moths, but can include other small insects such as dragonflies, mosquitoes, flies, crickets, and beetles. In a single meal the hoary bat can eat up to 40 percent of its weight. Their prime foraging time occurs in the late evening and due to their low frequency echolocation, most of it occurs over wide, open areas. Unlike other bats, hoary bats appear to fly with a very low echolocation frequency—think of a human with a strong glasses prescription driving at night without them.

This has come at a cost to these bats. Since they prefer open, wide hunting grounds and rely on poor senses while flying, they are easily obstructed when there are artificial objects in formerly open areas (such as large wind turbines in high meadows). Sadly, hoary bats are the species most frequently killed by wind turbines in North America, and make up 38 percent of bat fatalities at wind energy facilities in North America.

The proposed wind project outside Scotia presents obvious concerns, given its location near Humboldt Redwoods State Park (read more on page 14). Among the worst case scenarios: the project would have the potential to create a “population sink” for the western North American population of hoary bats. According to one recent study, impacts from wind energy projects are so great that the hoary bat population is expected to dip 90 percent in just 50 years.

Luckily there are measures that can be taken to reduce the risks posed by wind energy development on the species. Key among these is to curtail energy production during high risk periods, such as during migration or during nights with low wind speeds. Curtailment alone has the potential to reduce fatalities between 44-93 percent. Additional measures, such as acoustic deterrence, could further reduce potential fatalities.