FOOD-DEPENDENT BENEFITS OF HATCHING ASYNCHRONY IN AMERICAN KESTRELS FALCO-SPARVERIUS

Food supply and hatching asynchrony were manipulated for 90 broods of American kestrels (Falco sparverius) during 1989-1991. We measured the growth and mortality of nestlings within four treatment groups (async hronous, synchronous, food-supplemented, unsupplemented) to test the b rood reduction hypothesis of Lack (1947, 1954). Fledging success did n ot differ between synchronous and asynchronous broods when food was po or but consistent with the brood reduction hypothesis, nestlings died at a younger age in asynchronous broods. When food was supplemented, m ortality did not occur in the synchronous broods but youngest nestling s still died in asynchronous nests despite apparently adequate food fo r the brood. Oldest nestlings in asynchronous broods fledged with a gr eater mass than their younger siblings, also consistent with Lack's hy pothesis. Average nestling quality in synchronous broods was very depe ndent on food levels. Synchronous young that were supplemented were, o n average, the heaviest of any treatment group but young from unsupple mented synchronous broods were the lightest. Overall, patterns of mort ality and growth for kestrels support the brood reduction hypothesis w hen food is limited, but not when it is abundant. This food-dependent benefit of asynchrony in the nestling period is a prerequisite for fac ultatively adjusted hatching spans during laying.