Compensatory effect of egg size dimorphism on hatching asynchrony in Magellanic penguin

Egg laying is one of the most important phases in a female bird's breeding cycle. Its cost is high because eggs contain all the resources needed for the development of an embryo. Variation in size and quality of eggs can have important long-term consequences for offspring survival. Hatching asynchrony is known to influence sibling competition in many bird species. Last-hatched chicks will have a competitive disadvantage throughout the pre-fledgling period because they are smaller. The aim of this study was to analyze the effect of hatching asynchrony and egg size variation on the growth and fledging success of Magellanic penguin Spheniscus magellanicus chicks after disentangling the effects of parental condition. We simultaneously manipulated egg size dimorphism, hatching asynchrony and parental condition by performing a cross-fostering experiment, creating broods with controlled egg size dimorphism and hatching asynchrony in a colony of Magellanic penguins located in Isla Quiroga, Santa Cruz, Argentina. We found that hatching asynchrony had a negative effect on last-hatched chicks, but this disadvantage was mitigated by egg size dimorphism in their favor. Moreover, females in good condition invest more in second than in first chicks, which, added to a greater investment by foster fathers, leading to offspring fledging in good condition. On the contrary, for the first-hatched chicks, we found that body condition of the biological father was an important factor for their growth. We conclude that raising more than one chick seems to be a decision based on parental condition throughout the breeding season.