Determinants of Intraspecific Brood Amalgamation in Waterfowl

Abstract

-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. Received 27 October 1995, accepted 16 July 1996. AS A RESULT of intraspecific brood amalgamation, parents provide care to unrelated young in many species of mammals (Packer et al. 1992), birds (Eadie et al. 1988), fishes (Johnston 1994, Fraser and Keenleyside 1995), and insects (Mappes et al. 1995, Petanidou et al. 1995, Premoli and Sella 1995). Initially a puzzling case for classic evolutionary theory, intraspecific brood amalgamation can increase reproductive success of adoptive parents, adopted young, and/or genetic parents (Riedman 1982, Eadie et al. 1988, Sayler 1992). Among birds, intraspecific brood amalgamation arises in two ways. Prehatching brood amalgamation (preHBA) occurs when one female lays eggs in the nest of a conspecific that incubates the clutch and subsequently raises the offspring. Posthatching brood amalgamation (postHBA) occurs when the young from one family mix permanently with offspring from another conspecific family. The two types of intraspecific brood amalgamation are common in species that have self-feeding young, such as waterfowl. Nevertheless, the determinants of 1 Present address: Faculty of Veterinary Medicine, University of Montr6al, CP 5000, St-Hyacinthe, Quebec J2S 7C6, Canada. E-mail: beauchgu@ere.umontreal.ca intraspecific brood amalgamation in waterfowl are poorly known (Eadie et al. 1988, Rohwer and Freeman 1989, Afton and Paulus 1992, Sayler 1992). Various life-history and ecological determinants of intraspecific preHBA and postHBA in waterfowl have been suggested on the basis of comparative studies that commonly use individual species or genera as independent units of analysis. The validity of these claims is questionable, however, because phylogenetic information rarely is used. For example, it is conceivable that information from tribes with many species inflated some life-history and ecological correlations (Harvey and Pagel 1991). Consequently, I examined whether evolutionary transitions to intraspecific preHBA and postHBA in waterfowl are associated with certain life-history and ecological variables using the phylogenetic classification proposed by Livezey (1991; 1995a, b, c; 1996). Below, I review the non-phylogenetically based determinants of intraspecific brood amalgamation that have been suggested in earlier waterfowl studies. Determinants of preHBA. -Discussions of the determinants of intraspecific preHBA have focused on nest visibility, nest density, parental behavior, and reproductive effort. Nest visibility and nest density may reflect the ease with