Multiple Mechanisms of Reversed Hatching Asynchrony in Rockhopper Penguins

Abstract

Brood reduction in birds is frequently associated with hatching asynchrony, wherein incubation commences before clutch completion, causing last-laid eggs to hatch after earlier eggs. In crested penguins (Eudyptes spp.), second-laid eggs typically hatch before first-laid eggs, but the mechanisms behind this reversal are unknown. Through a multifactor field experiment with rockhopper penguins (Eudyptes chrysocome Forster 1781), I examined several factors that might influence incubation length : laying order (smaller first-laid eggs vs. larger second-laid eggs) ; incubation onset (exposure to immediate vs. delayed incubation) ; intraclutch dimorphism (incubated with an equal-sized vs. larger nest mate) ; laying date ; egg mass ; and egg composition (yolk and albumen content). Three of these factors - laying order, intraclutch dimorphism and laying date-contributed significantly to variation in incubation length ; first-laid eggs produced early in the season and incubated with a larger nest mate took longest to hatch. Differences in incubation onset did not contribute to incubation length when tested independently. The albumen content of both first and second eggs increased in proportion to fresh egg mass, but egg mass had no effect on incubation length. Although Eudyptes parents can potentially adjust the degree of hatching asynchrony through variation in egg position, the effects of laying date and laying order are probably beyond facultative control. Together, these effects may contribute to the unique reversal in both hatching asynchrony and egg-size dimorphism characteristic of crested penguins.