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

Based on more than 300 individually marked American Robins (Turdus migrator ius) and Brown Thrashers (Toxostoma rufum), I tested three hypotheses to ex plain 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 failu re increases reproductive effort by causing birds to renest, and this energ etic stress increases the probability of mortality; and (3) birds use a "de cision rule" based on prior experience to select nesting sites, such that i ndividuals that experience low reproductive success are more likely to move to an alternate breeding site, whereas birds that nest successfully are mo re likely to breed in the same site again. Birds subjected to experimental nesting failure returned at a significantly lower rate (robins 18%, thrashe rs 12%) than birds that nested successfully (robins 44%, thrashers 29%). Bi rds that nested more than once in a season returned at rates (robins 43%, t hrashers 21%) indistinguishable from birds that nested only once in a seaso n (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 dis persal in response to nesting failure.