No abstract

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.

Incubation prior to clutch completion may be adaptive if it maintains egg viability by inhibiting eggshell microbial growth, thus reducing the likelihood that the embryo becomes infected. To test this hypothesis, we examined the effect of partial incubation during egg laying on eggshell microbial loads in eastern bluebirds Sialia sialis breeding at a temperate‐zone site. We sampled eggshell microbes prior to and following four days of exposure to either partial incubation during the laying period or ambient environmental conditions without incubation (experimental eggs). Microbial colony counts declined significantly for eggs left in the nest during the laying period but did not vary significantly for eggs exposed to ambient conditions. Initial microbial loads were more similar to those previously reported from tropical than temperate environments, and microbes from potentially pathogenic groups were detected on 88% of first‐laid eggs on the day of laying. Egg viability was maintained when eggs were held indoors for four days without incubation but declined sharply thereafter. Our results suggest that partial incubation during egg laying may enhance egg viability in eastern bluebirds by reducing eggshell microbial loads; these effects appear stronger than those usually reported from the temperate zone.