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

Broods of Pied-billed Grebes, Podilymbus podiceps, breeding near Minnedosa, Manitoba (55°16′ N, 99°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 (, n = 20, 2 SE = 0.4) was higher during foraging bouts with below-average rates of food delivery by parents, as opposed to bouts ( 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 hierarchies 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 ...

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