Burrowing Owls on the Farallones??!
December 31, 2010
Yes, it’s true! There are burrowing owls on Southeast Farallon Island (aka, SEFI), and no, they are not an introduced species. Many people will be surprised to learn that these petite (10 inches tall and 1/3 pound) owls are here, because they are terrestrial owls, typically associated with expanses of flat grassland, open fields or lots, and medium-sized fossorial (burrowing) mammals such as ground squirrels or prairie dogs. SEFI is mostly rocky, devoid of burrowing mammals other than the introduced Siberian house mouse, is surrounded by ocean and nearly 30 miles from any expanse of land – hardly typical habitat for the burrowing owl. And yet they are here! The U.S. Fish and Wildlife Service and PRBO Conservation Science biologists have been monitoring the burrowing owls on the island for the last few years, and this year, they have brought on a graduate student to continue the owl monitoring and conduct a more in-depth look into the ecology of the owl. This student is Sara Lee Chandler, who is currently pursuing her Master of Science degree at San Jose State University.
The burrowing owl that occurs on SEFI is the western burrowing owl (Athene cunicularia hypugaea), a subspecies that ranges from Canada, through the Midwestern States of the U.S. and into Mexico. In California, the burrowing owl is a Species of Special Concern, and it is an endangered species in Canada. The burrowing owl population has long been in decline throughout its range – including within California – and many burrowing owl advocates within California are pressing for a more comprehensive conservation strategy and effort to protect the species from further decline.
A small number of burrowing owls make a seasonal appearance, arriving on SEFI from somewhere on the mainland in late September and sometimes overwintering as late as May. The burrowing owl does not breed on the island, but returns to the mainland to breed. One of the goals of monitoring the owls is to keep track of how long they stay on the island and where they are roosting. Sara conducts daily roost surveys of the accessible portions of the island (the seal crew conducts some of these surveys when she’s on break), scanning the nooks and crannies of the granite cliffs and slopes for an owl perched at its roost entrance, and keeping a keen eye on the ground to find pellets that have been deposited in the night. Many of the owls have been previously banded at SEFI and wear leg bands that uniquely identify them, which helps to keep track of which owl is at a particular roost.
The majority of the owls seem to prefer to roost in the innumerable rock crevices and fissures worn into the face of the island. Many of these crevices are used by breeding seabirds – such as Pigeon guillemot and Cassin’s auklet – in the summer. Sometimes burrows dug by Cassin’s auklets provide suitable habitat for the burrowing owl on the marine terrace. Sara collects the owl pellets at the roosts, which are regurgitated leftovers from an owl’s meal, and will analyze their contents to compare the composition of the owls’ winter and spring season diets. In general, however, it is known that owls are an opportunistic eater, feasting on the introduced house mice, but also preying upon songbirds, small seabirds (such as the ashy storm-petrel), beetles, and other terrestrial invertebrates on the island.
The burrowing owls, while relatively few in number, are a unique part of the ecology on SEFI. It is our hope that ecological studies of the burrowing owl on SEFI will contribute toward the knowledgebase of this charismatic animal.