Variation in the Size and Composition of Eggs of the European Starling

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Abstract
Between 1 and 5 eggs from each of 37 clutches (n = 88) of the European starling (S. vulgaris) were analyzed to determine the relative amounts of shell, yolk and albumen and the levels of water, lipid and nonlipid dry matter in these components. The yolk made up 17.7% of fresh egg mass. The albumen contained 89.8% water; yolks contained 55.7% water and 29.9% lipid. Amounts of ash and concentrations of cations in the yolk and albumen are reported. The mass of the shell varied in direct proportion to fresh egg mass, the amount of albumen varied in proportion to the 1.1 power of egg mass and the yolk in proportion to the square root of egg mass. Unlike the eggs of precocial species, both the yolk and lipid content of the egg were poorly correlated with egg mass (R2 = 0.22 and 0.14, respectively). A principal components analysis revealed that 90% of the variation in egg size and composition could be related to 3 factors roughly corresponding to egg size (51%), percent yolk (27%) and ratio of lipid to nonlipid dry matter in the yolk (14%). As in other passerines, between 50 and 80% of variation in most egg characteristics occurred among clutches rather than within clutches; composition of the yolk was least related to clutch. With respect to position in the laying sequence, only the percentage of water in the egg showed a significant trend, decreasing slightly from 1st to last. Because egg size is a poor predictor of yolk size, it may not necessarily provide a useful index to egg quality or be correlated with nestling growth and survival.

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