Effects of prior nesting success on site fidelity and breeding dispersal: An experimental approach

Abstract

Based on more than 300 individually marked American Robins (Turdus migratorius) and Brown Thrashers (Toxostoma rufum), I tested three hypotheses to explain low return rates of birds whose nesting attempts are unsuccessful: (1) birds with low reproductive success are low-quality individuals that are more likely to suffer mortality between breeding seasons; (2) nesting failure increases reproductive effort by causing birds to renest, and this energetic stress increases the probability of mortality; and (3) birds use a 'decision rule' based on prior experience to select nesting sites, such that individuals that experience low reproductive success are more likely to move to an alternate breeding site, whereas birds that nest successfully are more likely to breed in the same site again. Birds subjected to experimental nesting failure returned at a significantly lower rate (robins 18%, thrashers 12%) than birds that nested successfully (robins 44%, thrashers 29%). Birds that nested more than once in a season returned at rates (robins 43%, thrashers 21%) indistinguishable from birds that nested only once in a season (robins 36%, thrashers 23%). These results, as well as supplementary data, were inconsistent with hypotheses 1 and 2 and consistent with hypothesis 3. This study provides strong evidence that low return rates result from dispersal in response to nesting failure.