Intraspecific Variation in Hatching Asynchrony: Should Birds Manipulate Hatching Spans According to Food Supply?

Abstract

The adaptive significance of hatching asynchrony in birds has been the subject of considerable controversy. Numerous hypotheses have been proposed to explain hatching patterns, but few of these account for intraspecific variation in those patterns. I developed a mathematical model of facultative manipulation of hatching based on the brood reduction hypothesis and the assumption that hatching patterns have different fitness payoffs in good and bad food-years. I compared the productivity of facultative manipulation to the productivity of a single, fixed, hatching span. When food resources during the nestling period are partly predictable from those during the prelaying period, facultative manipulation appears advantageous in many types of environments. Predictability of food resources depends on the nature of the food supply, especially temporal patterns of abundance over the long- and short-terms. Correlation analyses showed that the small mammal prey of the American kestrel, a bird which practises facultative manipulation of asynchrony, were quite predictable during the summer. Generation times of a species in relation to the timing of fluctuations in food resources may also influence whether or not facultative manipulation evolves.