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Bird populations -- United States, Avian biology, Eastern Kingbirds


Observational and experimental procedures were used to evaluate the potential importance of brood size and weather on the evolution of clutch size in the Eastern Kingbird. Modal clutch size was three eggs, yet broods of four were most productive. Nestling size varied inversely with brood size, so that "nestling quality" was lower in broods of four than in broods of three. Asymptotic weight of nestlings in broods of three was directly and significantly correlated with ambient air temperature, followed by hatch order. Because larger broods were being fed during a period of relatively cool and wet weather, the effects of brood size were probably confounded by weather-induced changes in food availability, suggesting that the ability to produce surviving young successfully from large broods is dependent upon weather conditions during the nestling period. These observations match patterns seen in other aerial foragers and indicate that unpredictable changes in temperature and precipitation dramatically affect the ability of the adults to feed young. A high variance in fledging success from the largest broods is indicated and probably selects for a reduced number of offspring in the Eastern Kingbird. Trade-offs in reproductive effort and parental survival do not appear to be required to explain patterns of clutch size in the Eastern Kingbird or, probably, other aerial foragers.


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