THE SIGNIFICANCE OF THE COLOUR OF TURDINE EGGS

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Abstract
Summary: 1. In the Turdinae, colour of eggs is valueless as a guide to classification, since not infrequently a species has eggs that are unlike those of its congeners and like those of a species in a different genus.2. Instead, certain colours of eggs tend to be associated with certain types of nesting site; species nesting in deep holes tend to have immaculate white eggs; those in shallower holes and niches speckled white, immaculate blue or speckled blue eggs; those on the ground, amid herbage or on ledges obscured brown, grey or olive eggs; those in forks in bushes or trees blotched eggs, often with shadow‐marks, on a whitish or blue ground; those building domed nests immaculate or speckled, white or blue, eggs.3. The immaculate white eggs of hole‐nesters perhaps result merely from the absence of selection by predators, but more probably are adapted to increase the visibility of the eggs to the parents in a dim light. Obscured brownish or olive eggs are adapted for concealment on the ground or amid herbage. Between these two extremes is a series of species nesting in shallower holes, niches and recesses, in which the eggs are progressively more spotted the more open the nest, suggesting an adaptive gradient, determined primarily by the need for concealing coloration and perhaps also by the need for some degree of visibility to the parents. There is an alternative series in which the eggs are a deeper blue in shallower holes, possibly those in which the nest is screened overhead; immaculate blue eggs also occur in dense low bushes. The blotchings and shadow‐marks on eggs in nests in forks of trees are thought to be cryptic, but why some should have a whitish and others a blue ground colour is not understood.4. There is no reason to think that the eggs of small passerine birds are warningly coloured.