Avian egg size varies widely within populations and larger egg sizes reflect greater investment by the female parent. Whether, and how, the extent of egg size variation affects postnatal development, are important questions affecting our understanding of optimal resource allocation between pre- and post-hatching parental investment in altricial and semi-precocial species. Though much research has been carried out in the hope of answering these questions, the feeding ability of parents confounds the results. To verify a possible advantage of hatching from a large egg on chick growth, without the confounding effect of parental feeding ability, an artificial feeding experiment was carried out on the Rhinoceros Auklet Cerorhinca monocerata, a semi-precocial alcid species. Hatchlings from various sized eggs were reared with the same amount of food in separate cages. To verify the advantage of larger eggs, I tested whether the wing lengths of chicks from large eggs grew faster than those from smaller eggs; such growth is critical to chick survival at fledging. The hatchlings from large eggs were found to have greater mass, longer wings and better body condition than those from small eggs. The chicks from large eggs had greater growth only in their wings during the initial postnatal development. This accelerated growth resulted in them having longer wings at fledging. These results indicate that large egg size in the Rhinoceros Auklet provides a guarantee of chick growth and fledging success despite an unpredictable environment during the chick rearing period.
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