Brood size and hatching patterns in birds are widely believed to represent adaptations to food availability during the brood-rearing period. Experimental manipulations of brood size and hatching patterns have been widely employed to determine whether parents can raise larger or more synchronous broods, but concurrent manipulations of food abundance to test the supposed causal mechanism have been rare. I studied survival to fledging of 1,060 American Coot (Fulica americana) chicks from 99 broods for which brood size, hatching asynchrony, and food availability had been experimentally manipulated via inter-nest transfers of newly hatched young and provisioning of supplemental food. Survival of color-marked young was measured until 45 days posthatch using Cormack-Jolly-Seber mark—resighting models. Survival of offspring from unsupplemented broods declined linearly with experimental increases in brood size, and this decline was large enough to ameliorate any benefits to parents from larger broods. However, offspring survival was unaffected by experimental alterations of brood size in American Coots that received supplemental food, and supplemented pairs would have benefited from raising larger broods. Parents that produced larger clutches were more successful at raising large broods, consistent with the individual-optimization hypothesis. By contrast, observed hatching patterns were not optimal at promoting offspring survival, with both experimental increases and reductions in asynchrony leading to higher fledging rates. American Coot parents appeared to be adept at regulating food allocation among offspring with or without hatching asynchrony, which suggests that hatching patterns are most likely an artifact of selection for early onset of incubation.
Vol. 128 • No. 4
Vol. 128 • No. 4