Relative Effects of Hatching Order, Egg-Size Variation, and Parental Quality on Chick Survival in Common Terns

Abstract

ABSTRACr.-I examined the relative importance of factors influencing chick survival in manipulated and natural broods of Common Terns (Sterna hirundo). In nonmanipulated broods of three (modal brood size), egg mass decreased with laying order, siblings hatched asynchronously, and chick survival declined with hatching order. The ratio of a chick's mass to that of its siblings, at the time of brood completion, explained the greatest portion of variation in chick survival in these broods and in manipulated broods in which survival also varied with hatching order. Hatching order was an important component of mass ratio at brood completion. Egg mass did not influence survival in these brood types. In contrast, egg mass was important in manipulated broods in which chicks hatched synchronously and in natural broods of two, for which hatching order did not affect survival. Parental quality, as indicated by the number of a chick's siblings that survived, also had a positive effect on chick survival that was more important than the negative effect of increasing brood size. Length of the incubation period was positively correlated with size of last-laid eggs. My data did not strongly support the brood-reduction hypothesis of hatching asynchrony and egg-size variation. A smaller egg may benefit the last-hatched chick by reducing hatching asynchrony. Received 20 May 1992, accepted 29 November 1992.