Proximate and Ultimate Causation of Egg Size and the “Third-chick Disadvantage” in the Western Gull

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Abstract
It is generally observed in gulls (Larus spp.) that produce a typical clutch of three that the third- or last-laid egg is smaller and lighter than its earlier-laid counterparts. This typically results in the third chick hatching later, growing at a slower rate, and having a higher rate of mortality. This suite of factors has been described as the "third-chick disadvantage," and various functional interpretations have been suggested to explain its adaptive basis. We report on egg size, chick growth, and survival in a population of Western Gulls (Larus occidentalis) where the third-chick disadvantage appeared to be nonexistent. We suggest that functional interpretations of this phenomenon may be premature and that variation in egg size in gulls may simply be due to variation in female energy reserves, and that in colonies where food is abundant and nest density low, the third-chick disadvantage may be reduced or absent.