Producing extra eggs does not deplete macronutrient reserves in European Starlings Sturnus vulgaris

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Abstract
Recent studies have suggested that the size of avian clutches may be determined in part by the costs of egg production, which are thought to involve the depletion of endogenous nutrient reserves. I tested the hypothesis that producing extra eggs would deplete protein and lipid reserves in a passerine species, the European Starling Sturnus vulgaris. The first three eggs of clutches were removed on the days they were laid to induce females to lay extra eggs. Females responded to this manipulation by laying, on average, 1.3 eggs more than controls. However, based on the distribution of clutch sizes, it appears that some females responded to the removal of three eggs by producing more than one extra egg, whereas others responded by laying no extra eggs or by deserting. In unmanipulated females, the lean dry mass of the flight muscles and total body lipid were found to decline throughout breeding, an observation which is often interpreted as evidence that egg production depletes endogenous reserves. However, females experimentally induced to produce an extra egg did not have reduced stores of protein or lipid at clutch completion. No effect was observed in starlings even though this study had substantial (80%) power to detect a change in protein reserves of the same magnitude as that observed in a previous study of gulls. The decrease in flight muscle mass that has frequently been observed during breeding in passerines may not be due simply to mobilization of endogenous protein reserves.

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