Hatching asynchrony and food allocation within broods of Pied-billed Grebes, Podilymbus podiceps

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Abstract
Broods of Pied-billed Grebes, Podilymbus podiceps, breeding near Minnedosa, Manitoba (55.degree.16''N,99.degree.50''W), in the summer of 1984, hatched from five to eight eggs over 3-7 days. We found a negative correlation (partial r=-0.56, df=11, p < 0.05) between the length of the hatching interval and the number of chicks surviving to 35 days of age, after controlling for clutch size. The mean number of sibling aggressions (.hivin.x=1.6, n=20, 2 SE=0.4) was higher during foraging bouts with below-average rates of food delivery by parents, as opposed to bouts (.hivin.x=0.5 aggressions, n=18, 2 SE=0.2) with above-average rates. Also, larger chicks initiated and won more disputes over positions near adults (Fisher''s exact test, p < 0.05, n=40) than did their smaller sibs. Thus, size hierachies among siblings influenced food allocation within Pied-billed Grebe broods. Food distribution, however, was nonrandom for 12 of 18 foraging bouts in which there were no apparent size differences between attendant chicks. Signals of hunger status from begging chicks, and parental regulation of which chicks maintained positions near them, also affected food allocation among siblings. Finally, handling times of prey, captured by adults and fed to chicks, depended on brood age (partial r=-0.21, df=183, p < 0.05) after controlling for prey size. For Pied-billed Grebes, noticeable size hierarchies among chicks remained throughout most of brood development and may have been advantageous during times of low food availability, or may have enabled parents to forage more effectively on a wide range of prey types and sizes.