Hatching asynchrony in Eurasian kestrels in relation to the abundance and predictability of cyclic prey

Many hypotheses for hatching asynchrony within avian clutches centre around food supply during the laying or nestling periods, but food is measured in few studies. Patterns of clutch size and hatching asynchrony were analysed in the Eurasian kestrel [Falco tinnunculus (Linnaeus)] breeding in Finland, where small mammal populations show dramatic 3-year cycles. Data collected over 12 years showed that clutches were larger when food was more abundant in spring. While controlling for clutch size, hatching was more synchronous in the low phase of the vole cycle compared to the increase phase, contrary to conventional interpretation of the brood reduction hypothesis. The age of the female, but not the age of the male was associated with the degree of asynchrony. Paradoxically, numbers of voles were most stable in the 'low' phase, which may lead to a unique situation, where food is more predictable when scarce. Kestrels have small, synchronous clutches in the 'low' phase and large asynchronous clutches in the 'increase' phase. Based on results of this study, it is suggested that it is important to consider the predictability of food, and not only the mean abundance of food when evaluating hypotheses for hatching asynchrony. Patterns of clutch size and hatching asynchrony at the population level can be used to evaluate current energy-related hypotheses for asynchrony.