IS THERE A SEXUAL CONFLICT OVER HATCHING ASYNCHRONY IN AMERICAN ROBINS

Many hypotheses have been proposed to explain the occurrence of hatchi ng asynchrony in altricial birds. According to one hypothesis, parents would benefit from hatching asynchrony because the offspring would sp end less energy on sibling rivalry. Another hypothesis states that the re is a sexual conflict over hatching spread, with one parent trying t o minimize investment at the expense of its mate. The latter hypothesi s was suggested from a study where males survived better following asy nchronous than following synchronous hatching of the brood, with oppos ite results for females. I tested this hypothesis by manipulating hatc hing spread in another altricial bird with biparental care, the Americ an Robin (Turdus migratorius), and by observing food provisioning late in the nestling period. The hypothesis was supported because males se emed to contribute less, and females more, to asynchronous broods. Mal es did not take less care than females of the smaller nestlings within the broad, but when the brood had partially fledged it was mostly the female that fed the young that remained in the nest. Provisioning of the parents combined did not seem to be less with asynchronous than wi th synchronous hatching, lending no support to the sibling-rivalry red uction hypothesis.