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.

凤头企鹅属（Eudyptes）的鸟类具有一项独特的鸟类繁殖特征：其产下的首枚A卵重量仅为第二枚B卵的54%至85%。尽管该属7个物种的窝内卵大小二态性程度存在显著差异，但所有物种均存在典型的强制性育雏缩减现象，绝大多数成功出飞的幼雏均源自体型更大的B卵。长期以来，众多学者一直试图阐释凤头企鹅为何会演化并维持这种高度二态性的双卵窝繁殖策略，以及为何首枚产卵远小于第二枚，但直到近年才有人提出可检验的近因机制。对于多数凤头企鹅物种而言，雌性个体在返回繁殖群落的洄游途中便已启动卵子形成过程。在马可罗尼企鹅（Eudyptes chrysolophus）中，在陆地上的产卵前间隔更短（由此推测其洄游与卵子形成的重叠程度更高）的雌性个体，所产卵的二态性更强，这提示存在某种生理冲突可能限制了更早启动发育的A卵的生长。这种“洄游遗留效应假说（migratory carry-over effect hypothesis, MCEH）”已在新西兰坎贝尔岛的东部岩跳企鹅（Eudyptes chrysocome filholi）中得到检验：研究人员通过两年的野外监测，记录了植入应答器标记的雌性个体的抵达与产卵日期、体型参数以及卵重。结果显示，产卵前间隔更长的雌性，其卵窝的二态性更低，这与MCEH的预测相符。然而，对个体雌性的重复监测发现，不同年份间个体卵大小二态性的变异，与产卵前间隔的个体内变异并无关联。卵重以及相对次要的卵大小二态性，均为与体型和体质量高度相关的可重复性状。结合上述结果与对MCEH的详细考量，研究表明凤头企鹅的卵大小二态性不太可能由洄游遗留效应所导致。