Restoring a Balance Among Seabirds

Endangered Species Bulletin, Nov, 1998 by Joelle Buffa, Rick Morat

Biologists with the Point Reyes Bird Observatory have been conducting monitoring and research on the Farallon National Wildlife Refuge through a cooperative agreement with the FWS for over 25 years (roughly the life-span of an ashy storm-petrel), and they will monitor the success of the gull control project. The U.S. Department of Agriculture's Wildlife Services (formerly Animal Damage Control) provided technical advice on gull control methods and assisted with project design.

Farallon National Wildlife Refuge, a satellite of the more expansive San Francisco Bay National Wildlife Refuge Complex, is a group of rocky islands located about 28 miles (45 kilometers) west of San Francisco. The 211 acres (85 hectares) in the islands contain the largest seabird nesting colony south of Alaska, with approximately 200,000 individuals of 13 different species breeding on the islands each year. The refuge supports the largest breeding population of ashy storm-petrels (Oceanodroma homochroa), comprising 50 to 70 percent of the world population, and the world's largest breeding population of another bird, the western gull (Larus occidentalis).

So, what are managers to do when ecological relationships become strained among a few seabird species, namely western gulls, ashy storm-petrels, and Cassin's auklets (Ptychoramphus aleuticus)? Long-term monitoring recently revealed an alarming trend: ashy storm-petrel breeding populations declined 40 percent from 1972 to 1992 and Cassin's auklets declined over 60 percent between 1971 and 1989. Evidence pointed to western gulls as the culprits. The number of gull-killed petrels that island biologists were finding each year (about 2.5 percent of the population per year) approximated the observed population decline of ashy storm-petrels on the 120-acre (48 ha) Southeast Farallon Island (SEFI). Dead Cassin's auklets that have been regurgitated by western gulls are commonly found along island paths each morning. Western gulls apparently prey on the burrow-nesting seabirds, which are about the size of a robin, as the auklets return to incubate eggs and feed their chicks at dusk.

A Population Viability Analysis determined that the slow-reproducing, but long-lived, petrels couldn't survive this level of predation, and concluded that the species should be considered vulnerable. While not listed under the Endangered Species Act, the petrel is considered a species of concern. Both the refuge and the Point Reyes Bird Observatory, the refuge's cooperator in monitoring seabird populations and managing SEFI, agreed that something was needed to reduce gull predation on petrels and auklets. Biologists began exploring solutions and funding sources in early 1997.

First, what was it about western gulls that made them a problem? Although western gulls have always bred on SEFI, the distribution of nesting colonies has changed dramatically over the years. Previously, western gulls nested only on certain, mostly rocky, portions of SEFI, areas not favored by burrow-nesting auklets and crevice-nesting petrels. Gradually, however, the western gulls have expanded their nesting habitat and now nest all over SEFI, including in high quality auklet and petrel habitat, which is scarce on the island. Gull populations have benefited from feeding on mainland landfills some 25 miles (40 km) away, while changes in oceanic productivity have been unfavorable for seabird species such as auklets.

Second, what were the management options for reducing gull predation? Three potential solutions were culling, egg addling, and construction of physical barriers. After weighing the pros and cons, the Fish and Wildlife Service FWS) decided to try physical barriers first and researched design options from successful projects in other parts of the United States and Canada. Installing a system of overhead cables seemed promising, as gulls avoid landing and flying through parallel overhead wires, just as cattle tend to avoid walking over parallel rails.

In October 1997, refuge staff constructed two experimental gull exclosures to discourage western gulls from nesting on a portion of the marine terrace on SEFI. Each exclosure consists of 11 parallel overhead cables strung between paired poles, with cables elevated about 4 to 6 feet (1.2 to 1.8 meters) above the ground. In one exclosure, the cables are about 10 feet (3 m) apart (plot size = 30 meters square), and in the other, the cables are about 15 feet (4.5 m) apart (plot size = 50 meters square). The cable is made of a lightweight, pliable plastic material in order to minimize the risk of injury to flying birds. The project was funded with $6,000 from the FWS Coastal Ecosystems Program.

In April 1998, nesting boxes also were installed within the exclosure plots to entice ashy storm-petrels to recolonize the newly predator-free habitat. To promote recolonization, biologists played taped calls of petrels all night long between April and August on a specially designed solar-powered tape/ playback recorder purchased as part of the project. This technique of attracting seabirds is known in seabird biology terms as social attraction.