The Ecological Factors Selecting for Polygyny in Altrical Birds

Abstract

Elaboration of the Orians-Verner natural selection model for the evolution of polygyny leads to the expectation that the occurrence of territoriality in bachelor males will be indicative of the selective factors favoring polygyny in altricial birds. Bachelor males establish territories only if differential food availability between habitats has been an important factor favoring polygyny. Bachelor males do not establish territories when differential protection from predation afforded by potential nesting substrates has been the primary selective factor. Previously published evidence is consistent with these predictions. Vegetation structure as a defense against nest predation may be of paramount importance in determining habitat quality for colonial species; food availability should be a more important factor for non-colonial species. All species in which bachelor males defend territories are noncolonial, and nearly all species in which bachelor males do not defend territories are colonial. This correlation supports the conclusions derived from the model and suggests 2 ways that polygyny evolved in altricial birds. In colonial species polygyny apparently evolved because a limited availability of safe nesting sites can be defended by a fraction of the total male population. Many females can only occupy these sites by mating with already mated males. In noncolonial species polygyny apparently evolved because a combination of physiognomic differences and differential food availability between optimal and marginal habitats enables secondary (2nd-mated) females in optimal habitat to be as successful as monogamous females in marginal habitat. Polygyny probably evolved in a few species because females are unable to determine the mated status of males, and the Orians-Verner model is inapplicable in these cases.