Again, Cunningham is not correct in saying that sexual dimorphism is “virtually absent” in the Columbidæ. Few birds display so striking a sexual dimorphism as the Orange Dove (_Chrysœna victor_) of Fiji, in which the male is bright orange and the hen green. We have already cited the case of the curious sexually dimorphic red turtle-dove. Now, the courting attitudes and actions of this species are precisely the same as those of other allied turtle-doves; why, then, have these exercises caused only one species to become sexually dimorphic?
Our survey of the more important attempts which have been made to explain the phenomena of sexual dimorphism leads to the conclusion that these still require elucidation. We have weighed each theory in the balance and found it wanting.
The outstanding feature of sexual dissimilarity is the apparently haphazard manner of its occurrence.
We have already alluded to the case of the doves in India. In that country four species are widely distributed—namely, the Spotted Dove (_Turtur suratensis_), the Ring or Collared Dove (_Turtur risorius_), the Little Brown Dove (_Turtur cambayensis_), and the Red Turtle-dove (_Œnopopelia_ _tranqebarica_). The habits of all these four species appear to be identical, nevertheless in the first three the sexes show little or no dissimilarity in outward appearance, while in the last the sexual dimorphism is so great that the cock and hen were formerly thought to belong to different species.
Another very curious case is that of the South American geese of the genus _Chloëphaga_, in which some species, as the familiar Upland or Magellan Goose of our parks (_C. magellanica_), have the sexes utterly unlike, while in others, as the Ruddy-headed Goose (_C. rubidiceps_), they are quite similar to each other.
The ducks furnish us with another very good example of the apparently haphazard nature of sexual dimorphism. In the Common Mallard or Wild Duck (_Anas boscas_) the cock is far more showily coloured than the hen, but in all the species most nearly allied to it the males are as inconspicuous as the females, _e.g._ in the Indian Spotted-bill (_Anas pœcilorhyncha_), the Australian Grey Duck (_A. superciliosa_), the African Yellow Bill (_Anas undulata_), and the American Dusky Duck (_A. obscura_). As the dusky duck inhabits North America, where the mallard is also found, the case is particularly striking.