We investigated patterns and consequences of intraclutch egg-size variation in Magellanic Penguins (Spheniscus magellanicus). First-laid eggs were significantly larger than second-laid eggs, although the mean difference represented only 2% of an average egg's volume. The degree of intraclutch egg-size variation was similar among years and females of different ages. Intraclutch egg-size variation did not affect intraclutch differences in chick hatching weights or fledging success. We found no selective advantage for laying eggs of different sizes. Because both eggs have an equal probability of being lost, chance favors equal provisioning of eggs. Egg volume explained 35% of the variation in hatching weight but did not determine fledging success. Laying order, year, and female age were better predictors of fledging success than egg size. Factors such as laying and hatching order, parental quality, oceanographic conditions, fights, and predation are more important in determining chick survival than are differences in egg size.
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Vol. 107 • No. 4