Hatching asynchrony in the Eurasian kestrel Falco tinnunculus : an experimental test of the brood reduction hypothesis

Summary 1.  We tested the brood reduction hypothesis by manipulating hatching spans of Eurasian kestrel Falco tinnunculus (Linnaeus) broods during two low vole years (1996–97) in western Finland. In addition, half the broods in 1997 were food supplemented during the entire nestling period. 2.  Nestling mortality was high, occurring in 55% of nests, and the effect of manipulated hatching span on fledging success was more pronounced than either the effects of year or supplementary food. In contrast to the predictions tested, fledging success was higher in synchronous than asynchronous nests, both when food was limiting and when food conditions were substantially improved by food supplements. 3.  We found no evidence that brood reduction resulted in higher quality (body mass) of surviving young in low vole years. On average, fledglings were heavier at synchronous than asynchronous nests. This was only the case, however, when food was limiting. 4.  Most nestling mortality occurred when the chicks were 2 weeks old, and age of death did not differ between the brood types. Parents at synchronous nests fed their broods at a higher rate than at asynchronous broods, this divergence arising only in the latter part of nestling period. 5.  This is the first hatching span experiment where synchronous hatching seemed more profitable than asynchronous during conditions of both food scarcity and food abundance. The higher parental workload at synchronous nests could, however, counterbalance the benefits of synchronous hatching. 6.  Hatching in natural broods of Eurasian kestrel is more synchronous in low vole years compared to high vole years. The fact that synchronous broods out‐performed asynchronous broods in both ‘low’ years of the study is consistent with the facultative manipulations. The benefit of asynchrony in high vole years is still unclear.