Individual repeatability in laying behaviour does not support the migratory carry-over effect hypothesis of egg-size dimorphism in Eudyptes penguins

Penguins of the genus Eudyptes are unique among birds in that their first-laid A-egg is 54–85% the mass of their second-laid B-egg. Although the degree of intra-clutch egg-size dimorphism varies greatly among the seven species of the genus, obligate brood reduction is typical of each, with most fledged chicks resulting from the larger B-egg. Many authors have speculated upon why Eudyptes penguins have evolved and maintained a highly dimorphic 2-egg clutch, and why it is the first-laid egg that is so much smaller than the second, but only recently has a testable, proximate mechanism been proposed. In most species of Eudyptes penguins females appear to initiate egg-formation at sea during return migration to breeding colonies. In macaroni penguins E. chrysolophus, females with a shorter pre-laying interval ashore (and thus presumably greater overlap between migration and egg-formation) lay more dimorphic eggs, suggesting a physiological conflict may constrain growth of the earlier-initiat...