Determinants of intraspecific brood amalgamation in waterfowl

Intraspecific brood amalgamation in waterfowl can occur before hatching, when females lay eggs in the nests of conspecifics, or after hatching, when females abandon their young to the care of other conspecific families. Using phylogenetic information, I examined whether evolutionary transitions to intraspecific prehatching and posthatching brood amalgamation in waterfowl can be associated with certain life-history and ecological variables. Transitions to intraspecific prehatching brood amalgamation occurred more frequently in lineages carrying the colonial-breeding state and the cavity-nesting state. However, such transitions occurred equally frequently in lineages carrying different life histories as indexed by the ratio of clutch mass to female body mass. The results support the view that opportunities for successful amalgamation, which are thought to be higher in colonial-nesting and cavity-nesting species, act as a proximate determinant of prehatching brood amalgamation. Transitions to posthatching brood amalgamation occurred most often in lineages carrying the prehatching brood-amalgamation state, which suggests that amalgamation after hatching represents an extension of amalgamation before hatching that temporally increases the window of opportunity for brood mixing.