MULTIPLE INCENTIVES FOR PARENTAL OPTIMISM AND BROOD REDUCTION IN BLACKBIRDS

Abstract

Why do parent birds hatch their young asynchronously? This phenotypic handicap exacts a cost of reduced growth and elevated mortality from the last-hatched or “marginal” offspring, while conferring advantages to the more senior “core” brood. David Lack long ago proposed that marginal offspring allow parents to track resources that are uncertain at the time of clutch initiation. If food is insufficient for the entire brood, hatching asynchrony allows surplus marginal offspring to be culled from the brood efficiently. This in effect represents a secondary adjustment of clutch size. Today Lack's hypothesis remains a central, although controversial, component, of discussions of hatching asynchrony and avian brood reduction. Most field workers report results inconsistent with Lack's hypothesis (e.g., a failure of asynchronously hatching broods to produce more fledglings than do experimentally synchronized broods) and have offered a variety of alternative explanations. Previous workers have focused, with limited success, on single explanations for the complex phenomena of hatching asynchrony. Here we use parental optimism theory to study the secondary adjustment of clutch size in an integrated framework emphasizing ecological (tracking uncertain resources) and developmental (unpredictable survival, quality and/or sex of progeny) uncertainty. We present the results of a 6-yr study of Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), a marsh-nesting icterid. Marginal offspring that are the products of hatching asynchrony serve multiple functions simultaneously: in tracking resources that vary temporally; as a hedge against hatching failure; and in progeny choice via sex-biased brood reduction. The effect of the phenotypic handicap was diminished both by improving ecological conditions and by hatching failure/experimental clutch reduction. Marginal offspring represent high variance progeny, and synchrony represents a high variance strategy. Brood reduction occurred at low cost and was rendered more efficient by hatching asynchrony, particularly under conditions of stringency.