Data from: 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-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.冠企鹅属（Eudyptes）企鹅在鸟类中独树一帜：其首枚产出的A卵（A-egg）重量仅为次枚产出的B卵（B-egg）的54%~85%。尽管该属7个物种间的窝内卵大小二态性差异悬殊，但所有物种均普遍存在强制育雏缩减现象，绝大多数成功离巢的幼雏均源自体型更大的B卵。诸多学者曾围绕冠企鹅属企鹅如何演化并维持高度二态性的双卵窝，以及为何首枚产出的卵远小于次枚卵这两个问题展开推测，但直至近期才有人提出可验证的近因机制。 对于冠企鹅属多数物种而言，雌性个体似乎会在返回繁殖群的迁徙途中于海洋中启动卵的形成过程。在马可罗尼企鹅（E. chrysolophus）中，在岸上的产卵前间隔更短的雌性（推测其迁徙与卵形成过程的重叠程度更高）所产的卵二态性更强，这表明生理冲突可能会限制更早启动发育的A卵的生长。 该迁徙携带效应假说（migratory carry-over effect hypothesis, MCEH）在新西兰坎贝尔岛的东部跳岩企鹅（E. chrysocome filholi）群体中得到了检验：研究人员在两年时间内记录了植入应答器标记的雌性个体的抵达日期、产卵日期、体型大小以及卵重。正如MCEH所预测的那样，产卵前间隔更长的雌性所产的窝卵二态性更低。然而，对雌性个体的重复测量结果显示，不同年份间个体内的卵大小二态性变异，与个体内产卵前间隔的变异并无关联。卵重以及程度稍弱的卵大小二态性，均为与体型大小和体质量相关的高度可重复性状。结合上述研究结果与对MCEH的详细审视，可认为冠企鹅属企鹅的卵大小二态性不太可能由迁徙携带效应所导致。