With regard to sexual colour, the dull summer plumage which characterises so many ground-breeding birds is all the more remarkable as they are the mates of males for the most part distinguished by unusual brilliancy. The few exceptions to this rule are of the most interesting character, and go eminently to prove it. In these exceptions it happens that the female birds are more brightly plumaged than the males. But in nearly the whole of the cases this remarkable trait comes out--that the male actually sits upon the eggs. Now this fact more than any other would seem to indicate that the protection afforded by obscure colouring is directly intended to secure the bird's safety during the most critical period of its life-history. And it has been seen that the law of protective colouring most influences those birds which breed on the ground. One remarkable instance of this may be given, that of the dotterel, a bird already mentioned. This is a species of our own avi-fauna, one which breeds on the summits of the highest mountains. Mr. Gould has remarked that dotterel have not unfrequently been shot during the breeding season with the breast bare of feathers, caused by sitting on the eggs; and the writer knows of his personal knowledge that the shepherds on the Cumbrian mountains occasionally kill dotterel on the actual nest, and that these almost invariably turn out to be males. In winter the colouring of the sexes is almost identical; but when the breeding season comes round the female dons a well-defined, conspicuous plumage, while it is found that the dull-coloured male alone sits upon the eggs. Mr. Wallace has pointed out that bee-eaters, motmots, and toucans--among the most brilliant of tropical birds--all build in holes in trees. In each of these cases there is but little difference in the plumage of the sexes, and where this is so the above rule is almost invariable. Again, our native kingfisher affords an illustration. The orange-plumaged orioles have pensile nests, which is a characteristic of the order to which they belong, most of the members of which are conspicuous. Bird enemies come from above rather than below, and it will be seen that the modifications referred to all have reference to the upper plumage.