Hybridization and reproductive performance in gulls of the Larus glaucescens-occidentalis complex

The Glaucous-winged Gull (Larus glaucescens) and the Western Gull (L. occidentalis occidentalis) hybridize along the Pacific Coast from Cape Flattery, Washington, south through central Oregon. A hybrid index, based on plumage and soft part colors, was used to study the distribution of morphotypes (e.g., pure and hybrid birds), mating patterns and reproductive performance at eight sympatric and two allopatric colonies. Mate choice appears to be weakly positive-assortative. In 1989 in the hybrid zone, L. o. occidentalis experienced significantly greater reproductive performance, measured as clutch size and egg volume difference, than either L. glaucescens or mixed pairs containing at least one hybrid individual. Although the hybrid zone has expanded somewhat in recent times, the midpoint of the zone and the relative proportion of morphotypes has remained stable. The hybrid zone is situated on a marine-ecotone. Within the hybrid zone, relative reproductive performance of L. o. occidentalis and hybrids may shift between years, whereas L. glaucescens may experience poor reproductive performance in all years. The selection-hybridization balance hypothesis may best explain the L. glaucescens-occidentalis hybrid zone.