This study compares the plumage of a single Eurasian White-throated Dipper Cinclus cinclus with that of three Common (European) Blackbirds Turdus merula (all road casualties). As expected, the Dipper had far more down and contour feathers (3,322) than the Blackbirds (mean 1,704), with Dipper down and contour feathers being significantly shorter than those of Blackbirds. However, contrary to published information, the plumage of dippers is not exceptionally dense; the mean follicle density of the Dipper was 59.2 follicles cm-2, that of the Blackbirds 58.6–61.7 follicles cm-2. Instead, the enhanced insulation of diving Dippers stems from a) much more extensive plumage (the apterylae between feather tracts are fully feathered, whilst naked in Blackbirds), b) substantial down cover of head (absent in Blackbirds), c) extensive down cover of wings. Much of the higher number of feathers in the Dipper is made up of contour feathers rather than down/semiplume feathers (the ratio between Dipper contour and down/semiplume feathers, 3.35:1, was similar to that of Blackbirds, 3.55:1), indicating an insulative function for contour feathers as well as down. Ultrastructural investigation showed that Dipper breast and back down was substantially finer (more complex) than in Blackbirds, but leg down, and contour feathers of the two species were indistinguishable.
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Vol. 32 • No. 1