JOINT LAYING IN BUCEPHALA DUCKS - PARASITISM OR NEST-SITE COMPETITION

Joint laying by buffleheads Bucephala albeola and Barrow's goldeneyes B. islandica can be explained by assuming that all females start laying with the intent to incubate. When 2 birds lay in the same nest, one bird incubates the joint clutch; birds excluded once and continuing to lay may incubate a partial clutch, or another joint clutch; birds excluded once and continuing to lay may incubate a partial clutch, or another joint clutch, or be excluded again. Birds that have been excluded from incubation twice may "dump' 1 or 2 eggs that they cannot resorb, as a last resort. -from Author