Los Farallones

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

Migratory Bats on the Farallon Islands

By | September 24, 2015

SEFI Aerial_East Side_JohnWarzybok

During the fall, we study many species of animals as they stopover at Southeast Farallon Island to find food and shelter to help them prepare for their migrations. For example, several aquatic migrants stopover at the Farallon Islands to take advantage of the nutrient rich waters, such as white sharks, blue whales, humpback whales, gray whales, and several species of dolphin. We also monitor many species of insect, such as butterflies, moths, dragonflies, and damselflies, as they migrate out over the ocean and require land to rest before they continue on to places unknown. The most abundant and conspicuous species that we study are birds. Occasionally, when the weather is just right, we wake up to find large numbers of songbirds covering the island. These are the weather conditions that fall biologists dream of. But this weather also brings another aerial migrant to the Farallones, bats.

Since the mid-1960’s, fall biologists on Southeast Farallon Island have conducted daily surveys for songbirds and bats. Both tend to congregate in or around the four trees on the island, so we spend quite a bit of our time around those trees. Most of the birds are fairly easy to find as they flit about trying to find food among the branches. The bats, though, are more difficult to find as they cling, or roost, motionless under the branches. Since bats are nocturnal, there is little reason for them to fly around during the day. Our daily bat survey consists of scanning the undersides of branches in hopes of seeing a little bat hanging face-down. We spend about 15 minutes under each tree.
The hoary bat (Lasiurus cinereous) is the most common species of bat that we encounter at the island. This species is named such because of the white tipped hairs covering their back. They differ from most of the other North American bats in that they are highly migratory, with many wintering in southern California, Mexico, and Central America. As is typical of other species in the genus Lasiurus, they mostly lead solitary lives, and they use trees for roosting. Because most other bats roost in caves, the bats in this genus are frequently referred to as “tree bats”. Since bat monitoring began, we have seen hundreds of hoary bats roosting in the trees on the island, with 101 counted on a single day in 1987!

Unfortunately, the numbers of hoary bats we see each fall began plummeting around 2008. There are two possible reasons for this and both probably play a part. The first is that their population has been adversely affected as wind turbines inadvertently kill them during migration. Apparently mammal lungs are highly susceptible to collapsing when exposed to a large pressure differential, such as when a massive wind turbine blade whips past. The other reason, though, which may have had an even larger impact, was the replacement of the rotating beam on the lighthouse to a weaker pulsing flash.


Another species of bat that is closely related to the hoary bat and also migrates out to the Farallon Islands is the western red bat (Lasiurus blossevillii). This species, though, is much less common out here, and prior to this year, there were only 11 records. On 20 September, while conducting an area search for birds, we found one roosting in the “Burning Bush”, a large mirror bush (Coprosma repens) native to New Zealand. The last record before this was in 2001, when one was seen flying about at dusk as it dodged the repeated passes of a peregrine falcon.

Western red bats are distributed throughout the western U.S., with the exception of the Great Basin, and their range extends southward all the way to central Argentina. Red bats in the northern part of their range and at higher elevations migrate south and towards the coast for winter. Suitable habitat for this species can be found in well-developed riparian forests, where they may roost under a dense canopy of leaves. They feed on a variety of insects, with moths being preferred. This mostly solitary species only seeks company of its own kind in the fall, when they mate. After mating, the female delays fertilization until spring so that it can give birth in the summer. Amazingly, litter size can be as high as four, which the female must carry around until the pups are old enough to fly. Unfortunately, western red bat populations are also in decline, as much of their riparian habitat has either been destroyed or degraded. Hopefully this little guy makes it back to the mainland after it has rested a while during its migratory stopover.

Posted by Jim Tietz, fall biologist

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