Brood size manipulations in herring gulls

Abstract

Herring gull broods were artificially increased to four and five chicks while others were reduced to one chick. The growth rates of chicks and chick weights at fledging were similar among all control and experimental broods. The experimentally increased brood sizes had a slightly higher fledging success than control broods, and the fledging success of one-chick experimental broods was higher than that of one-chick control broods. Thus, chick survival was not reduced in larger brood sizes and the number of young fledged per pair of adults increased with brood size. Further. chicks in large broods were at no weight (or viability) disadvantage despite the presence of a larger number of brood mates. We conclude that food was not limiting either the growth rate or fledging success of chicks from broods larger than the modal clutch size, and suggest that parents used local, abundant, artificial food sources in addition to their natural food supply.