Life-history variables evolve in response to cost-benefit trade-offs. For birds, larger eggs are thought to be beneficial for development of offspring but are energetically costly to produce. Further, egg-size dimorphism within or between clutches can vary with proximate and ultimate causes. We undertook a correlational study to evaluate parental investment in eggs by Magellanic Penguins (Spheniscus magellanicus) and how it affects the growth and survival of nestlings in Puerto Deseado, Argentina, over 3 years. We evaluated the variables that affected egg-volume and yolk-area (using a nondestructive technique), and determined the effects of egg-volume and yolk-area on growth and survival of young. Females in good body-condition laid larger second eggs and, in good years (i.e. years of high reproductive success in the colonies of the study area), yolk-area of second eggs was larger than that of first eggs. We found a positive association between eggvolume and nestling body-size and yolk-area was positively related to nestling survival. Our results suggest that the size of eggs within clutches varied with year and female body-condition. Moreover we demonstrate for the first time that yolkarea is a strong predictor of nestling survival in Magellanic Penguins.

The Magellanic penguin Spheniscus magellanicus is a top predator and a major consumer of marine resources in the Patagonian Continental Shelf and worldwide. It is the most migratory of the Spheniscus penguins, but until recently, its migration route was only partially known. Our aims with regard to the Magellanic penguin breeding on Isla Quiroga, Argentina, were: 1) to compare the distribution during the winter period between sexes and 2) in case that habitat is segregated, to evaluate possible proximate causes like competitive exclusion and/or habitat specialization. In March 2017 and 2018, prior to migration, we equipped 26 penguins with MK3/4 geolocators, which were recovered in late September/early October. Penguins dispersed northwards up to 29°S and southwards up to 56°S, near the Beagle Channel, being distributed over 2158 ± 50 km of latitudinal range and showing a large‐scale distribution pattern across longitudes in waters within the continental shelf. We found a longitudinal segregation between sexes in their winter grounds, with males closer to the 200‐m isobath and further from the coastline than females. Morphological and physiological differences between sexes might allow males to dive deeper at a lower cost than females, which could result in a more offshore path for males. This spatial segregation was supported by bulk blood stable isotope values and estimated isotopic niche, which differed between males and females at their arrival from winter breeding grounds, but not while penguin breed and behave as central place foragers in the colony. For the first time in this species, we combined two independent tools, geolocation and stable isotope data, and find that sexual segregation in habitat use could potentially generate an optimal winter foraging strategy for both sexes, precluding potential intersex competition for food.

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