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

A number of hypotheses for hatching asynchrony suggest that the size h ierarchy among nestlings produced by hatching asynchrony is adaptive a nd confers benefits to parents. We assessed the costs and benefits of asynchronous hatching in the Greenrumped Parrotlet (Forpus passerinus) , a small Neotropical parrot that hatches large clutches very asynchro nously. We manipulated eggs to create broods of four, six, or eight yo ung that hatched synchronously or asynchronously. In a second experime nt, we tested whether food limits offspring survival by experimentally feeding later hatched young in large asynchronous broods. We also exa mined the premise that food varies unpredictably by sampling seeds thr oughout several breeding seasons. Experimentally synchronized broods g enerally fledged as many or more young than asynchronous broods. Synch rony particularly outperformed asynchrony in broods of eight, where fo od demands should have been greatest. Nestlings had a higher probabili ty of hedging from synchronous broods than from asynchronous broods, f rom small rather than medium or large broods, and if they were early h atched rather than later hatched. Most mortality in asynchronous brood s occurred within 12 d of hatching, and a significantly greater propor tion of later hatched chicks died with empty crops than did early hatc hed chicks. Later hatched chicks grew more slowly than their earlier h atched nestmates, but at fledging they were as heavy or heavier than e arlier hatched chicks. Chicks from asynchronous broods were slightly h eavier at hedging than synchronous chicks, but there was no correlatio n between hedging mass and the likelihood of being resighted in subseq uent years. Cormack-Jolly-Seber model estimates revealed no significan t differences in annual survival rates between young fledged from sync hronous and asynchronous broods. Female chicks hedged 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 hedge than chicks from broods that fledged all their yo ung. 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 a nd asynchronous broods showed no significant differences in the likeli hood, timing, or success of their next breeding attempt. A marginally higher proportion of last-hatched chicks that received supplemental fo od survived to fledging than last-hatched control chicks, but feeding had no effect on penultimate chicks. Seed densities showed a high degr ee of autocorrelation over spans of 30-50 d. Asynchronous hatching app ears to result in the mortality of the smallest young, due in part to the inequitable distribution of food among nestmates, rather than to f ood limitation, and as a direct result of the size disparities among n estmates. Thus, parrotlet parents appeared to derive no detectable sho rt- or long-term benefits from the staggered hatching of their young t hrough increased nestling growth and survival, reduced parental effort s, or increased parental survival. Although other adaptive benefits fr om hatching asynchrony are possible that were not tested directly in t hese experiments (e.g., ''insurance'' that some nestlings will survive ), they seem insufficient to account for the extreme hatching asynchro ny observed in the parrotlet. Instead, benefits to egg survival derive d from the early onset of incubation may offset the costs of asynchron ous hatching.