Fall on the Farallon Islands may bring up
exciting ideas about songbirds, seabirds, cetaceans, or pinnipeds, but
Southeast Farallon Island’s most abundant species each fall is the non-native
house mouse (Mus musculus). In 2010, population estimates recorded house
mouse density to be ten times greater than densities reported on the mainland
or on other islands. House mouse populations on SEFI are cyclic, peaking in the
fall and plummeting in late winter and early spring. This cyclic pattern is the
foundation of an interesting food web quandary involving two SEFI natives that
are also California Species of Special Concern: Burrowing Owls (Athene
cunicularia) and Ashy Storm-Petrels (Oceanodroma homochroa).
Burrowing Owls are small, long-legged raptors
that migrate to SEFI each fall. From as early as the late 19th century up until
the present day, Burrowing Owls have been observed on SEFI. They are
generalists that feed primarily at night on rodents, invertebrates, and birds.
In the fall, house mouse populations increase
on Southeast Farallon Island at the same time that Burrowing Owls are arriving.
Based on Burrowing Owl pellet dissection, we can see that in the fall the
majority of their diet biomass comes from mice. As the mouse population
declines towards the end of winter, our research has found that Burrowing Owls
switch to preying on Ashy Storm-Petrels which arrive mid-winter to breed. In
the 2010-2011 season, a graduate student from San Jose State University did a diet
study on Burrowing Owls providing evidence of this prey switching mechanism
happening among SEFI’s Burrowing Owls.
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This figure from Sara Chandler’s thesis work shows biomass of four
different prey items in Burrowing Owl diets observed through dissection of
collected pellets from September 30, 2010 to May 15, 2011. The switch from
using mice as the main diet source to Ashy Storm-Petrels over time is clearly
evident.
Switching to eating Ashy Storm-Petrels is
problematic. The worldwide population of Ashy Storm-Petrels numbers
approximately 10,000, and half of those birds breed on SEFI. This small seabird
is slow-breeding, long-lived, and can only produce one chick each year. When
several Burrowing Owls overwinter and each depredates dozens of Ashy
Storm-Petrels, the long-term impact on the breeding population is significant.
Ashy Storm-Petrel adult at its nest. Photo by
Annie Schmidt.
In order to monitor the fall and winter population
of Burrowing Owls on SEFI, Point Blue Conservation Science has had an owl
intern survey for Burrowing Owls each fall. In 2007, we began to capture and
band Burrowing Owls, in addition to daytime surveying, to get better population
estimates.
SEFI provides an ideal habitat for Burrowing
Owls, which roost in rocky crevices as well as Rhinoceros and Cassin’s Auklet
burrows. Burrowing Owls are incredibly cryptic, blending in very well with
their surroundings. Each day, the owl intern searches these rocky crevices and
burrows for owls and their regurgitated pellets. If an owl is found, we make an
effort to identify the individual by the unique combination of numbers and
letters on its leg band. These pellets are later dissected to get more information
on owl diet over time.
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Burrowing Owl in front of
its rocky crevice roost. Photo by Jim Tietz
The house mouse is the most abundant species on
the island in fall. Photo by Jim Tietz.
If mice are eradicated from SEFI, it is likely
that the overwintering Burrowing Owl population would decline. Owls that
currently arrive in the fall find plenty of mice to eat and therefore have
reason to overwinter. However, if the owls arrived in fall to a mouse-less
island, the biomass provided by the invertebrate prey would be insufficient to
sustain them through the fall and early winter. The owls would be forced to
migrate elsewhere in search of a more abundant prey base. Burrowing Owls that do
not overwinter are not preying on Ashy Storm-Petrels. Currently, the USFWS is
developing a proposal to eradicate the house mouse from the Farallon Islands,
which, if successfully implemented, would likely be an effective long term
benefit for the declining Ashy Storm-Petrel population.
So far during the fall of 2015, we have
documented 23 Burrowing Owl arrivals and have banded 19 of them. One owl
observed had been banded in 2013. In the past two months, 53 Burrowing
Owl Pellets have been collected. Currently, there are at least 4 Burrowing Owls
still on the island that may attempt to overwinter. Since 2007, the number of
arrivals each year has averaged around 17 Burrowing Owls. An exception to this
average was in 2012 when there were 54 individuals. We will continue to survey
for Burrowing Owls for the years to come in order to collect data on their life
histories and the impacts they have on Southeast Farallon Island’s ecosystem.
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An unbanded Burrowing Owl stands about 10 inches
tall. Photo by Jim Tietz.
A just-banded Burrowing Owl about to be
released. Photo by Natalie Okun.
Figure
1 comes from: Chandler, S. L. Burrowing owl (Athene cunicularia) diet and abundance at a stopover wintering ground on Southeast
Farallon Island, California (Unpublished master's
thesis). San Jose State University, San Jose, California.
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