THE GOOD, THE BAD AND THE UGLY - LACK BROOD REDUCTION HYPOTHESIS AND EXPERIMENTAL-DESIGN

Adaptive brood reduction, as originally proposed by David Lack and as revised by later workers, represents a simple mechanism to counter stochastic variation in ecological conditions: parents lay an optimistic clutch as a hedge against uncertain food, trimming brood size downward if food is short. Hatching asynchrony, which facilitates swift brood reduction, may also bear the unwanted cost of reduced survival of marginal offspring when food is abundant. Strong tests of the brood reduction hypothesis require explicit recognition of the conditional nature of these costs and benefits and must include three steps: (1) exact specification of where the threshold for brood reduction lies; (2) measurement of the costs and benefits of a brood reduction strategy under ''good'' and ''bad'' conditions; and (3) estimation of the frequency of ''good'' and ''bad'' conditions. Indirect measures of ecological conditions, such as the relative reproductive success of the study population in different years, are unsatisfactory for two reasons: (i) it is unclear when the brood reduction threshold has been crossed, and (ii) nestlings may starve for reasons other than extrinsic food limitation (e.g., sibling depotism). Heretofore, studies of avian brood reduction have routinely failed to provide an independent and explicit measure of food conditions rendering the results ambiguous. An appropriate experimental design that circumvents this problem is outlined here, and allows strong tests of Lack's hypothesis.