Lack's brood reduction hypothesis and avian hatching asynchrony: What's next?

Forbes recently argued that extant attempts to test Lack's brood reduction hypothesis for asynchronous hatching were of limited value. He argued that the hypothesis had to be tested in a two-step process: first, by testing whether brood reduction is adaptive, second, by testing whether asynchrony pays, provided the first test is fulfilled. Here, we focus on how this protocol can be adopted in empirical work. First, we identify several theoretical limitations to Forbes's approach: (1) it is unclear whether the brood reduction threshold should be tested using synchronously or asynchronously hatched broods, (2) individuals are different and thus have individual ''brood reduction thresholds'', and (3) unpredictability may occur at different levels. Second, me discuss a main practical difficulty in testing the brood reduction hypothesis, related to the fact that asynchrony may carry a cost when conditions are good. When this is the case, tests must be carried out under a wide range of conditions, and during many years. Such a testing programme may turn out an almost insuperable task and Lack's hypothesis may be practically untestable using the traditional approach of recording the quantity of fledglings. Instead, we suggest that focus in future research on the issue is turned to consequences of different hatching patterns with respect to offspring quality. Recent evidence suggests that asynchrony serves to ensure offspring quality instead of quantity (the Offspring Quality Assurance Hypothesis), and may also lead to less variable recruitment (bet-hedging).