Why Do Birds Lay Eggs in Conspecifics’ Nests?

Abstract

Conspecific brood parasitism (CBP) resembles interspecific brood parasitism, except that parasitic females lay eggs in the nests of conspecifics. CBP is a female alternative reproductive behavior, and understanding its evolution requires a life history approach. We review studies that investigate life history aspects of CBP. One or more life history components have been examined for 56 species, 49 of which are relevant to the adaptive basis of CBP. Various approaches have been used to detect CBP, but there is increasing reliance on molecular methods. Molecular methods are not always foolproof, and false exclusions are incorrectly interpreted as CBP; future studies should employ multiple lines of evidence. Two types of parasites have been documented, often in the same species: non-nesting females without their own nest in a given year and nesting females that lay some of their eggs parasitically. There is no evidence for lifelong professional brood parasites; non-nesting females apparently adopt parasitism to make the best of a bad job, although we lack a clear understanding of the specific constraints that prevent these females from nesting or the costs that make restraint a better option. Parasitism by nesting females is common, surprisingly so in the precocial waterfowl, and is only rarely associated with breeding interruption or nest loss. Nesting females appear to use parasitism to enhance their reproductive output, but for most species it is not clear why females don't lay the additional eggs in their own nest. A full understanding of this form of brood parasitism will require a better understanding of clutch size constraints.