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EcoNews Creature Feature: Hoary Bat

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Hoary Bat on a branch. Photo by Tom Benson.

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 an especially unique bat due to the fact that it migrates and hibernates, when most bats do one or the other. This strange trait has befuddled scientists but one of the potential possibilities for this behavior comes from 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. They are one of the only species of bats that hibernate in the open, such as on tree branches. This could explain why they are 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 of bats not yet seen elsewhere on the planet. There are so many hoary bat that this may mean that Humboldt Redwoods draws bats from all over western North America.

The diet of the hoary bat primarily consists of moths, but can include other small insects, such as dragonflies, mosquitoes, flies, crickets and beetles. They are an important predator of insects and a successful one at that, in a single meal the hoary bat can eat up to 40% of its weight. Their prime foraging times occur in the late evenings 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 frequency of echolocation—think of a human with a heavy prescription driving at night without their glasses.  

This has come at a cost to these bats.  Since they prefer open, wide hunting grounds and rely on very little 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, with 38% proportion of bat fatalities at wind energy facilities in North America being hoary bats.

The proposed wind project outside Scotia presents obvious concerns, given the location near Humboldt Redwoods State Park. 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 bats population is expected to dip 90% 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%. Additional other measures, like acoustic deterrence, can further reduce potential fatalities.