Reproductive separation and isolating mechanisms between sympatric dark phase and light phase western grebes aechmophorus occidentalis
Auk 96(3): 573-586
ISSN/ISBN: 0004-8038 Accession: 006317362
Studies of dark- and light-phase western grebes A. occidentalis during 1975-1977 revealed highly significant assortative mating by the color phases. Of 1185 pair observations made during 2 yr in Utah [USA], only 1.2% represented mixed pairs. The expected frequency of mixed pairs assuming random mating was 33%. Only 2 mixed pairs were noted in over 600 independent pair observations in California and Oregon. Mixed pairs with broods represented 0.25% of 766 broods surveyed. Nest initiation dates were significantly different between color phases in 1975 and 1976. Plumage development of captive chicks revealed striking differences; black crown feathers emerged 30-40 days later on light-phase chicks. Morphologically, only total culmen length for females differed significantly between color phases. Analysis of spatial distribution clearly indicates that light-phase birds are nonrandomly distributed among and within winter and summer flocks and among and within nesting colonies. The data from Utah and California reveal that dark- and light-phase western grebes behave as separate biological species. Isolating mechanisms may involve a combination of inter-colorphase recognition and spatial segregation.