Ecological isolating mechanisms based on differences in size or breeding season operate in many groups of related sympatric seabirds, but not in the alcid species (Aves: Fam. Alcidae) found together in Washington State. At least three of the six coexisting species have similar diets, and all six breed at the same time of year. They differ, however, in their foraging zones at sea and appear to have reduced interspecific competition by this means. Diverse aspects of the breeding biology, behavior morphology and ecology of these alcids can be explained by reference to two sets of selective factors–interspecific competition for food and predation upon the eggs, chicks and adults. Species that fish inshore guard their chicks in exposed cliff sites grouped into ubiquitous but small colonies. They carry fish to chicks singly and frequently throughout the day. These young develop rapidly to about one—third adult weight, at which time they quit the nest site. At the other extreme, species that feed far offshore provide burrows for the chick and nest in large colonies on only a few off—shore islands. Chicks are visited at night by adults whose bills are adapted for carrying many fish. These chicks grow slowly and reach close to adult weight before leaving the nest. These patterns are repeated with considerable fidelity in the six—species alcid community of Northern Iceland, but southern hemisphere seabirds are, in general,differently organized.