Nesting female lesser snow geese ( Anser caerulescens caerulescens ) usually adopt eggs that are laid adjacent to their nests by potential intraspecific nest parasites. The host female rolls the parasite’s egg up into the nest, using the same behavior patterns used to retrieve her own eggs if they are displaced from the nest. The reproductive consequences of adopting eggs are unclear. We consider three selective scenarios that might maintain adoption behavior in geese: (1) egg adoption is of no reproductive consequence to the host; (2) egg adoption augments host reproductive success relative to that of unparasitized nests; and (3) egg adoption is making the best of a bad situation, once a female’s options are constrained by the presence of an egg adjacent to the nest; we also consider (4) the possibility that adoption, if not concordant with the selective regime, is maladaptive. Nest parasitism is costly to the host, making hypotheses 1 and 2 unlikely. However, adopting eggs significantly decreases the risk of total nest failure during laying, more than offsetting other probable host costs. This is consistent with hypothesis 3. Experiments show that geese have limited abilities to retrieve eggs, which accounts for most cases of nonadoption of nearby eggs. We conclude that adoption of eggs is an adaptive trait, a form of nest protection. Adopting eggs is the best option within the species’ repertoire to the threat to nest survival created when a parasite lays an egg next to the nest. [Behav Ecol 1991;2:181-187]
Copyright (C) 1991, International Society for Behavioral Ecology
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