Is There a Sexual Conflict over Hatching Asynchrony in American Robins?

Abstract

-Many hypotheses have been proposed to explain the occurrence of hatching asynchrony in altricial birds. According to one hypothesis, parents would benefit from hatching asynchrony because the offspring would spend less energy on sibling rivalry. Another hypothesis states that there is a sexual conflict over hatching spread, with one parent trying to minimize investment at the expense of its mate. The latter hypothesis was suggested from a study where males survived better following asynchronous than following synchronous hatching of the brood, with opposite results for females. I tested this hypothesis by manipulating hatching spread in another altricial bird with biparental care, the American Robin (Turdus migratorius), and by observing food provisioning late in the nestling period. The hypothesis was supported because males seemed to contribute less, and females more, to asynchronous broods. Males did not take less care than females of the smaller nestlings within the brood, 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 with synchronous hatching, lending no support to the siblingrivalry reduction hypothesis. Received 14 October 1996, accepted 31 March 1997. THE YOUNG OF MOST ALTRICIAL BIRDS do not hatch simultaneously but over a period of one or more days. Many hypotheses have been proposed to explain hatching asynchrony (Magrath 1990, Stoleson and Beissinger 1996). For instance, the cost to the parents may be reduced because less energy is spent on sibling rivalry (the Sibling Rivalry Reduction Hypothesis; Hahn 1981), and any offspring mortality that occurs may take place at an early stage of breeding so that little energy is wasted (Lack 1954). Hatching asynchrony also may help to spread out the peak demand for food of individual offspring (Hussell 1972, Mock and Schwagmeyer 1990). Evidence suggests that parents need to work less with asynchronous than with synchronous hatching in some species (Fujioka 1985, Mock and Ploger 1987) but not in others (Bryant and Tatner 1990). The best way of assessing the costs of breeding would be to follow the subsequent survival and fecundity of the parents. When this was done in a study of Blue Tits (Parus caeruleus), there was no evidence that parents benefitted from hatching asynchrony (Slagsvold et al. 1994, 1995). In the study of Blue Tits, data for the two parents were combined. However, males and females seemed to differ in their re' E-mail: tore.slagsvold@bio.uio.no sponses, males having improved survival with asynchronous hatching, females with synchronous hatching. We therefore suggested that a sexual conflict exists between the sexes over the degree of hatching spread, with one parent trying to minimize investment at the expense of its mate (the Exploitation of Mate Hypothesis; Slagsvold et al. 1995). In Blue Tits, the conflict apparently is won by the female because hatching is rather synchronous despite a large clutch size. The female alone incubates, and by delaying incubation until most eggs have been laid, she can reduce the hatching span. Why would a female benefit from synchronous hatching of her brood? One possibility is that with synchronous hatching, the male may contribute more because otherwise the whole brood would starve to death. Breeding success may be more variable with synchronous than with asynchronous hatching (Amundsen and Slagsvold 1997) such that synchronous broods may be more sensitive to the amount of male parental care. The male may invest less if the brood contains some small, undernourished young with low survival prospects. Relative to the female, then, he may give greater priority to molting and territory defense than to parental care (e.g. Svensson and Nilsson 1997). In addition, males may be less certain of paternity of later-hatched young and hence reduce investment in these offspring. In support of these