9520846 Many bird species that feed their young in the nest lay eggs that hatch asynchronously, with the first laid eggs hatching from two days to two weeks before the last laid eggs. This study tests two long-standing hypotheses that seek to explain this phenomenon. The first suggests that asynchrony increases males' contribution to reproduction and thus reproductive success of a pair, by prolonging the time that males feed females and young. The second suggests that asynchrony reduces peak energy demands, by spreading the cost of raising offspring over a longer time. Although these hypotheses differ in their details, both assume that hatching asynchrony evolved in response to food limitation during the breeding season and that it functions to reduce parental effort. Falsifying these hypotheses would mean that breeding is not invariably constrained by food availability, which would alter our understanding of the factors that limit avian reproductive success. This research will test these hypotheses by measuring the metabolic rate of free-living green-rumped parrotlets that are rearing either synchronous and asynchronous broods containing either 4 or 8 chicks. Green-rumped parrotlets exhibit the most extreme degree of hatching asynchrony found in birds, yet are capable of rearing synchronous and asynchronous broods with equal success, making them ideal subjects for this project. Field metabolic rate of parents feeding young will be determined using the doubly labeled water technique, a method that employs isotopically labeled water to determine directly metabolism of free-living animals. The results will increase our understanding of the way that costs of reproduction affect breeding systems, and the extent to which energy demands have played a role in the evolution of avian life histories. It will also expand our knowledge of tropical parrots, on e of the most threatened groups of birds, and provide baseline data that may aid future conservation efforts.
