Hatching asynchrony reduces the duration, not the magnitude, of peak load in breeding green-rumped parrotlets (Forpus passerinus)

The peak load reduction hypothesis suggests that hatching asynchrony in alt ricial birds is adaptive because it reduces parental workload during the mo st energetically costly time in brood rearing. By staggering the ages of th eir offspring, parents may ensure that all nestlings do not reach maximum e nergy demand simultaneously. To test the hypothesis, we used the doubly lab eled water technique to measure the energy expenditure of green-rumped parr otlets (Forpus passerinus) that reared experimentally manipulated synchrono us and asynchronous broods. Peak metabolic rates of the two experimental gr oups did not differ, but parents of asynchronous broods metabolized signifi cantly less energy than did parents of synchronous broods throughout the fi rst half of the brood-rearing period. Our results suggest that hatching asy nchrony in parrotlets substantially shortens the temporal duration of high brood energy demand, but does not reduce the magnitude of peak energy deman d.