Alternation of clear-cut colour patterns in Corvus crow evolution accords with learning-dependent social selection against unusual-looking conspecifics

摘要

The typical crows (Corvus) are mostly all-dark, blackish birds, but there are several examples of contrastingly two-toned plumage, the pale tone varying from grey to pure white. In the Eurasian crow complex, formed from west to east by black Corvus corone corone, black-and -grey Corvus corone comix and black Corvus corone orien-talis, and in the African complex of black-and-white Corvus albus and black Corvus edithae, the existence of hybrid zones shows the phylogenetic closeness of the morphs withineach complex. Recent molecular studies have confirmed these relationships and shown C. corone (orientalis) to be close to black-and-white Corvus pecto-ralis, a southeastern Asian morph (Haring et al. 2012), and in Wallacea, black Corvus unicolor is close to black-and-white Corvus typicus (Jensson et al. 2012), although hybrids have never been reported in these two cases. All the two-toned patterns produce a superficially similar pied appearance, with a wide pale area centred on the anterior part of thetrunk and contrasting with the glossy black of the rest of the bird, including the face. However, the colour of the feather bases varies regionally; those of Eurasian birds are grey, those of African birds are white, and those of Wallacean birds are dark-grey, in my opinion a first suggestion of the different origins of these pattern alternations. Based on their phylogenetic trees, Haring et al. (2012) proposed that pied plumage appeared several times independently during crow evolution and that reversals to all-dark colour have also occurred. In support of the existence of such to-and-fro changes (each time constrained by intervening evolution), as far as it is true that ontogeny recapitulates phylogeny, I would add that Corvus dauuricus is usually pied in juvenile plumage, virtually all-dark in the first winter (although the two-toned pattern is still evident in close view), and later pied again (Cramp & Perrins 1994). Repeated gain and loss of colour patterns during phylogeny may depend on simplethreshold mechanisms in birds (Price & Pavelka 1996).