HATCHING ASYNCHRONY, BROOD REDUCTION, AND FOOD LIMITATION IN A NEOTROPICAL PARROT

Abstract

A number of hypotheses for hatching asynchrony suggest that the size hierarchy among nestlings produced by hatching asynchrony is adaptive and confers benefits to parents. We assessed the costs and benefits of asynchronous hatching in the Green-rumped Parrotlet (Forpus passerinus), a small Neotropical parrot that hatches large clutches very asynchronously. We manipulated eggs to create broods of four, six, or eight young that hatched synchronously or asynchronously. In a second experiment, we tested whether food limits offspring survival by experimentally feeding later hatched young in large asynchronous broods. We also examined the premise that food varies unpredictably by sampling seeds throughout several breeding seasons. Experimentally synchronized broods generally fledged as many or more young than asynchronous broods. Synchrony particularly outperformed asynchrony in broods of eight, where food demands should have been greatest. Nestlings had a higher probability of fledging from synchronous broods than from asynchronous broods, from small rather than medium or large broods, and if they were early hatched rather than later hatched. Most mortality in asynchronous broods occurred within 12 d of hatching, and a significantly greater proportion of later hatched chicks died with empty crops than did early hatched chicks. Later hatched chicks grew more slowly than their earlier hatched nestmates, but at fledging they were as heavy or heavier than earlier hatched chicks. Chicks from asynchronous broods were slightly heavier at fledging than synchronous chicks, but there was no correlation between fledging mass and the likelihood of being resighted in subsequent years. Cormack-Jolly-Seber model estimates revealed no significant differences in annual survival rates between young fledged from synchronous and asynchronous broods. Female chicks fledged from synchronous broods were recruited into the study population at a lower rate than those from asynchronous broods. Older chicks from reduced broods were less likely to fledge than chicks from broods that fledged all their young. Parents of large synchronous and asynchronous broods provisioned their young at similar rates and did not differ significantly in their subsequent survival. Females that raised experimentally synchronous and asynchronous broods showed no significant differences in the likelihood, timing, or success of their next breeding attempt. A marginally higher proportion of last-hatched chicks that received supplemental food survived to fledging than last-hatched control chicks, but feeding had no effect on penultimate chicks. Seed densities showed a high degree of autocorrelation over spans of 30–50 d. Asynchronous hatching appears to result in the mortality of the smallest young, due in part to the inequitable distribution of food among nestmates, rather than to food limitation, and as a direct result of the size disparities among nestmates. Thus, parrotlet parents appeared to derive no detectable short- or long-term benefits from the staggered hatching of their young through increased nestling growth and survival, reduced parental efforts, or increased parental survival. Although other adaptive benefits from hatching asynchrony are possible that were not tested directly in these experiments (e.g., “insurance” that some nestlings will survive), they seem insufficient to account for the extreme hatching asynchrony observed in the parrotlet. Instead, benefits to egg survival derived from the early onset of incubation may offset the costs of asynchronous hatching.