Effect of initial asymmetry in Magellanic penguin chick's performance. A cross-fostering experiment.

When chicks in a brood hatch asynchronously, the first to hatch gets fed sooner, creating a hierarchy of asymmetries in the brood. In species that suffer from brood reduction, this asymmetry can facilitate the death of one chick during periods of low food availability. The aim of this study was to analyze the effect of initial asymmetries on the growth and survival of Magellanic penguin Spheniscus magellanicus chicks, after disentangling the effects of parental condition, during four breeding seasons. We manipulated chicks' asymmetries within the brood by performing a cross-fostering experiment, creating broods with different weight ratios in a colony located in Isla Quiroga, Santa Cruz, Argentina. We found that weight ratio determines the asymmetry within the brood throughout the entire breeding season, and that final asymmetry also depends on the study season. Chicks that hatch last have a greater chance of survival when the weight ratio with their older sibling is closer to 1. Parental body condition affects chick growth but not asymmetry. To conclude, parents do not reverse the effect of initial asymmetry in post-hatching stages, even during breeding seasons of high reproductive success.