| Summary: | Niche overlap and resource partitioning have seldom been investigated in the marine habitats of seabirds but are obvious determinants of community and population status. We investigated interspecific differences in densities and spatial aggregations of seabirds during summer (June–August) off southwest Vancouver Island, British Columbia. Two 300-m-wide parallel transects were sampled along 66.6 km of shoreline in 1993–1996, centered 200 m and 600 m from shore. Analysis focused on the threatened Marbled Murrelet (Brachyramphus marmoratus) and three other fish-eating alcids. Densities of Marbled Murrelets were among the highest reported at sea in the species' range; they were concentrated close to shore (>75% within 0.6 km and virtually all within 2.0 km of shore), usually in water <20 m deep, and showed strong fidelity to certain sectors along the coast from year to year. Common Murres (Uria aalge) and Rhinoceros Auklets (Cerorhinca monocerata) were not as closely associated with nearshore habitat as Marbled Murrelets, and their longshore distribution also differed from that of the latter. Neighbor-K analysis showed that Marbled Murrelets were usually segregated from Common Murres and, to a lesser extent, from Rhinoceros Auklets, but the latter two species were usually aggregated together. Marbled Murrelets also left the area after breeding, whereas numbers of Common Murres and Rhinoceros Auklets increased from June through August. Pigeon Guillemots (Cepphus columba) showed variable distributions and no consistent associations with other alcids. Marine distributions of Marbled Murrelets were markedly different from those of other fish-eating alcids, and we discuss this in relation to possible interference competition from the larger Common Murres and Rhinoceros Auklets.Coexistence et répartitions spatiales de Brachyramphus marmoratus et autres alcidés au sud-ouest de l'île de Vancouver, en Colombie Britannique
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