Los Farallones

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

Conservation Conundrum: The interaction of Burrowing Owls, Ashy Storm-Petrels, and House Mice

    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 arrivin
g. 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.

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. 

  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.

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.

Posted by Natalie Okun, fall Burrowing Owl intern