Abstract
Within a colony of Fieldfares Turdus pilaris L. different clutch sizes (4-7 eggs) varied greatly with respect to average egg weight, and clutches with a medium number of eggs had heaviest eggs. Large (in terms of wing length) females bred at greater heights than did small ones, and the large females also laid large eggs. No significant correlation was obtained between weight of female and mean egg weight per clutch or clutch weight. In colonies where the nests were situated close to each other the egg weights were relatively low. The biomass of lumbricids in the birds' feeding areas (the bulk of the food) was positively correlated with mean egg weight in the colony.
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