So also with the birds themselves, it is difficult to see why the cock Song-Thrush should be protectively coloured, but the cock Blackbird conspicuous on account of his intense black plumage. I am not aware that the former assists in incubating the eggs, and that the latter does not perform any such office. It is obvious that certain birds have little or no need of protectively coloured plumage. Some are naturally protected by their size and strength, e.g. the Swan; others by size combined with gregarious habits, as the Rook and Heron; or, by these defences combined with great powers of diving, as the Cormorant and Guillemot. Birds, except very small ones, which nest in holes can dispense with protective colours. It may fairly be said that the position and structure of the nests regulate the colouring of the birds themselves as well as their eggs, and that wherever there is no special need for sombre shades of plumage, conspicuous or bright hues prevail.
I. Of birds which make open nests, either (a) both sexes are protectively coloured; or (b) the hen so coloured and the cock more showy.
It will be readily seen that small birds, and birds frequenting very exposed places, specially need protection. It is such birds which have both sexes protectively coloured, e.g. Song-Thrush, Hedge-Sparrow, Lark, smaller Game-birds, Rails, Plovers, Sandpipers. Certain species of the last-named group have the sexes different.
That the hen should be sombre but the cock conspicuous is not surprising in large birds, such as Ducks and the larger Game-birds. But in a less degree it holds good also in the case of many small birds, such as the Blackbird, Blackcap, Wagtails, some of the Finches, and Buntings; but in all these (except the Blackbird) the colouring of the upper parts tends to harmonize with their surroundings.
Among the Plovers and Sandpipers the Dotterel (Charadrius morinellus) and Phalarope (Phalaropus hyperboreus) may be mentioned as exceptional, for with them the female is more brightly coloured than the male. The explanation lies in the fact that these males perform the duties of incubation.
Most sea-birds are equally showily coloured in both sexes. They nearly all have pure white under parts, which strongly
