| Summary: | 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-initiated A-egg. This migratory carry-over effect hypothesis (MCEH) was tested in eastern rockhopper penguins E. chrysocome filholi on Campbell Island, New Zealand, by recording the arrival and lay dates, body sizes, and egg masses of transponder-tagged females over two years. Females with longer pre-laying intervals laid less dimorphic clutches, as predicted by the MCEH. However, repeated measures of individual females revealed that within-individual variation in egg-size dimorphism between years was unrelated to within-individual variation in pre-laying interval. Egg masses, and to a lesser extent egg-size dimorphism, were highly repeatable traits related to body size and body mass. These results and a detailed consideration of the MCEH suggest that egg-size dimorphism in Eudyptes penguins is unlikely to be caused by a migratory carry-over effect. Morrison 2016 Eastern Rockhopper Penguin laying data All data used in Morrison 2016 (Journal of Avian Biology doi:10.1111_jav.00740). Morrison penguin lay data.csv
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