Asynchronous hatching and food limitation: a test of Lack's hypothesis

-Lack's (1954, 1968) hypothesis that asynchronous hatching of altricial birds is an adaptive response to unpredictable food shortages during the breeding season was examined in the highly granivorous Zebra Finch (Poephila guttata). I compared growth and survival of nestlings in asynchronous and artificially created synchronous broods reared under food-limited and food-abundant conditions in an aviary. I also examined the role of parental experience on survival and growth of nestlings. There was no differential mortality of Zebra Finch nestlings due to either asynchrony or food abundance. Young in abundant food treatments grew more rapidly, however, than those in food-restricted treatments. Heaviest Zebra Finch nestlings in a brood grew more quickly than their lightest siblings when food was limited, supporting Lack's hypothesis. Further, differential survival of light and heavy siblings occurred when food was abundant, suggesting that asynchronous hatching can be maladaptive under some ecological conditions. Nestlings reared by inexperienced parents suffered greater mortality and slower growth when food was abundant than nestlings raised by experienced parents. Prefledging mass was correlated with size at adulthood. Received 25 November 1987, accepted 17 September 1987. LACK (1954, 1968) proposed that asynchronous hatching in altricial birds is an adaptive response to potential unpredictable food shortages. According to this hypothesis, a disparity in the size of young at the completion of hatching results in a hierarchy among siblings, and the youngest sibling receives less food. Brood reduction by starvation of the youngest chick may occur during times of food scarcity, and result in a higher probability of survival for the remainder of the brood than if all young were fed equally (Ricklefs 1965). Few studies have tested Lack's hypothesis experimentally while documenting or manipulating food abundance (e.g. Slagsvold 1982,1985; Bengtsson and Ryden 1983; Fujioka 1985; Mock and Ploger 1987). In this study I examined the effect of hatching asynchrony on nestling Zebra Finches (Poephila guttata) reared in captivity under conditions of food limitation and food abundance. I simulated synchrony in broods by manipulating eggs before incubation and was thus able to compare survival and growth of "synchronous" broods with "asynchronous" broods. I examined three predictions derived from the hypothesis that when food is scarce, asynchronous hatching facilitates differential growth I Present address: Institute for Environmental Studies, University of Washington, Seattle, Washington 98195 USA. and mortality of young, providing growth or survival benefits to older siblings. First, when food is limited, late-hatched nestlings of asynchronous broods are more likely to die or grow more slowly than their older siblings than are last-hatched nestlings of synchronous broods. Second, heaviest nestlings of asynchronous broods grow faster than those of synchronous broods when food is limiting. Furthermore, one would expect that asynchronous hatching is not disadvantageous when food is abundant. Third, there are no differences in growth and survival of nestlings by rank in the brood hierarchy in asynchronous or in synchronous broods when food is abundant. I also estimated how parental experience influences survival and growth of nestlings and the relationship between growth rates, prefledging mass, and size at adulthood.