The Distribution of Pigeon Guillemots (Cepphus columba) Breeding on the West Coast of Vancouver Island, British Columbia, in 1989

Abstract

In June and July 1989, we surveyed adult pigeon guillemots (Cepphus columba) associated with potential nesting habitat along the west coast of Vancouver Island, B.C. We recorded 856 birds on 54 islands. Three islands accounted for 70% of the total, but 85% of nesting aggregations held 1-10 birds. No breeding sites were found on Vancouver Island shorelines, probably due to a lack of suitable nest-sites inaccessible to ground predators. On occupied islands, a variety of nest-site types were used, largely reflecting their availability. Numbers of adult pigeon guillemots were correlated significantly with numbers of apparently occupied nests of other surfacenesting birds, that is pelagic cormorant (Phalacrocorax pelagicus), glaucous-winged gull (Larus glaucescens), and black oystercatcher (Haematopus bachmani). A seasonal movement to inshore waters, away from the exposed western coastlines of Vancouver Island in winter, probably reduced the effect on this species from the Nestucca oil spill off northwestern Washington in the 1988-89 winter. Pigeon guillemots (Cepphus columba) are inshore diving seabirds, which nest in cavities or burrows along most coastlines between southern California and northwestern Alaska, usually on small islands or on large cliffs, in sites relatively inaccessible to mammalian ground predators (Ewins 1993). The majority of the North American population is thought to breed in Alaska and British Columbia, yet there have been few extensive surveys there (Sowls et al. 1978, Rodway 1988, Emms and Morgan 1989). Pigeon guillemots are highly vulnerable to oil pollution because they spend most of their lives in the water (King and Sanger 1979, Piatt et al. 1990, Ewins 1993). On Vancouver Island they have been found to accumulate a wide range of environmental contaminants in body tissues, including relatively high concentrations of mercury (Noble and Elliott 1986). They have potential as effective indicators of ecosystem health in inshore waters, and this has been recognized in Alaska following the Exxon Valdez oil disaster (Piatt et al. 1990, K. Kuletz, pers. comm.), and recently in Washington's Puget Sound Ambient Monitoring Program (Mahaffy and Divoky 1993). Traffic of oil tankers and other shipping is heavy off the west coast of British Columbia's Vancouver Island, and the Nestucca oil spill off Gray's Harbor, Washington, during the 1988-89 winter, killed approximately 56,000 seabirds off Vancouver Island and Washington (Rodway et al. 1989, Burger 1992). Both pigeon guillemots and black guillemots (C. grylle) are difficult to census accurately (Ainley and Boekelheide 1990, Ewins et al. 1993). Attendance at guillemot colonies varies dramatically in relation to time of day, weather, sea conditions, tidal state, and stage of the breeding season (Petersen 1981, Cairns 1984, Ewins 1985, Nelson 1987, Vermeer et al. 1 993a). It is difficult to assess population trends from most previous census data because surveyors have not allowed for variation in these factors; most population data has been collected during surveys of other seabird species, conducted at various times of day and at different stages of the breeding season. Furthermore, guillemot nests are hard to find (Ewins 1993). Systematic counts now concentrate on adults frequenting colonies in the early stages of the breeding season, usually in the early morning and around high tide (Ewins 1985, Vermeer et al. 1993a). Despite these censusing problems, a general idea of breeding range and the relative importance of colonies in an area can be obtained by