Los Farallones

Dispatches from Point Blue’s field station on the Farallon Islands National Wildlife Refuge

Bats on Southeast Farallon Island

The morning of October 14, 1987 on Southeast Farallon Island was like something out of a bat biologists’ dream. To the biologists that lived on the island, it was just one heck of a “bat day”. Although hoary bats (Lasiurus cinereus) had consistently appeared on the island each fall since at least the mid 1960’s, that morning resident biologists saw something unprecedented—101 hoary bats hanging in trees, bushes, and other secluded nooks around the island. Such a landfall of bats may not sound unusual to those unfamiliar with their secretive lives, but to researchers and naturalists who study bats, and hoary bats in particular, the October 14th event is mind-boggling. Biologists can focus an entire career on studying bats and go a lifetime without seeing a hoary bat roosting in the wild, let alone a large group of them. Now island biologists were seeing over 100 in the same place at the same time! Something unusual happens on the Southeast Farallon Island.

Hoary bats differ from most of the 44 other bat species occurring in the continental U.S. in that they migrate across the entire continent of North America each year, roost in trees year-round, and tend to live solitary lives. That’s right; they usually live solitary lives, except during migration. California is an important wintering area for hoary bats, and the island bats are probably southbound migrants that veer off course from Point Reyes as they make their way down the Pacific Coast. Most tend to show up on the island during fall nights with overcast skies, low winds, and low levels of moonlight, and we suspect they are wandering too far from shore and visually cuing in on the light atop the island in an effort to find safe haven from the sea ( Southeast Farallon Island is the only place in North America where we know that hoary bats consistently show up during fall migration, the only place where they have been roosting in anything other than small family groups (> 4 individuals), and the only place where daytime mating has been seen. Add migratory bats to the list of incredible natural events that happen each year on the island.

PRBO and U.S. Fish & Wildlife biologists began cooperating with USGS bat researcher Paul Cryan in 2003 to try and learn more about the hoary bats on the island. For the past two years we have been deploying an automated bat detector on top of the lighthouse each fall to record the calls of hoary bats during their island stopovers. Visitation was sporadic last year, with only a handful of bats appearing in the trees during the day, but earlier this fall there was a landfall of at least 10 hoary bats during the day and the bat detector recorded more bat calls during the two adjacent nights than we have seen before. We are now analyzing those calls to try and figure out what the bats might be doing around the lighthouse at night, where they have previously been seen circling, sometimes in fairly large numbers. Echolocation calls should give us some indication of whether the bats are feeding during their migration stopover, or interacting in some other way, such as emitting social calls associated with mating and flocking behavior.

Why is all of this important? At about the turn of the millennium hoary bats began taking the lead role in a serious conservation issue facing bats—collisions with wind turbines. Although we are not yet sure exactly why it is happening, bats are colliding with industrial-scale wind turbines all over the world, sometimes at estimated rates as high as thousands of individuals per wind energy facility per year. We have never seen anything like it, as bats rarely collide with other anthropogenic structures. Alarmingly, hoary bats compose more than half of the bat fatalities at wind turbines in North America and they consistently show up beneath turbines at nearly every wind energy site adequately monitored for bats. It is unclear how many hoary bats might be out there roaming the continent, so it is difficult to assess the cumulative impacts of wind turbines on their populations, but it is entirely possible that turbines are increasing mortality to a level beyond what their relatively slow reproductive rate (~2 young per year) can offset. Migratory bats that concentrate in certain areas during migration are particularly susceptible to these types of threats.

How does Southeast Farallon Island fit into the picture? Aside from being near an important wintering area and the only place we know where hoary bats are consistently seen during migration, hoary bats appear on the island during a critical time—fall. The vast majority of bat fatalities at wind turbines, and particularly fatalities of hoary bats, occur during late summer and early fall. This led researchers studying the problem to conclude that something important is happening during fall migration in the lives of hoary bats, and therein lays the key to solving the puzzle of their mysterious susceptibility to turbines. Explanations for the susceptibility of bats to wind turbines span the board, ranging from random collisions of large flocks to bats being attracted to turbines as roosting, feeding, or mating sites. Whatever the cause, we hope that by continuing to take advantage of the incredible fall visitation of hoary bats to Southeast Farallon Island,we will move closer to explaining and hopefully solving this pressing conservation problem. Stay tuned for more bat blog news in the coming months and years! For additional info on the subtleties of bats, contact Paul Cryan (