The colors of birds are in many cases perhaps connected with the position and mode of construction of their nests.  Thus, we know that hen birds are generally less brightly colored than the cocks, and this is partly, perhaps, because bright colors would be a danger to the hens while sitting on their eggs.  When the nest is placed underground or in the hole of a tree, etc., we find it no longer to be such an invariable rule that the hen bird is dull-colored; but, on the contrary, she is then often as gaily colored as the male.  Such, for instance, is the case with the hen kingfisher, which is one of the brightest of British birds and one of the very few which make their nests underground; the hen woodpecker, which is also gaily colored and builds in hollow trees, forms a second instance.

In the few cases where the hens are as conspicuously colored as the cocks, and yet the nest is open to view, we generally find that the hens are strong, pugnacious birds, and well able to defend themselves.  There are even instances, though these are comparatively rare, in which the hens are more brilliantly colored than the cocks; and it is an interesting fact that it is then the cocks, and not the hens, which hatch the eggs.

It therefore seems to be a rule, with very few exceptions, that when both the cocks and hens are of strikingly gay or conspicuous colors, the nest is such as to conceal the sitting bird; while, whenever there is a striking contrast of colors, the nest is open and the sitting bird exposed to view.

Again, most fishes are dark above and pale below.  This points to the same fact, for when one looks down into the dark water, the dark color of their backs renders them the less easy to distinguish; while, to an enemy looking up from below, the pale belly would be less conspicuous against the light of the sky.  Those fishes which live deep down in the depths of the ocean present no such contrast between the upper and under surface.  Many of the smaller animals which live in the sea are as transparent as glass, and are consequently very difficult to distinguish.

It is sometimes said that if animals were really colored with reference to concealment, sheep would be green, like grass.  This, however, is quite a mistake.  If they were green they would really be more easy to see.  In the gray of the morning and the evening twilight, just the time when wild animals generally feed, gray and stone colors are most difficult to distinguish.  Sheep were originally mountain animals, and every one who has ever been on a mountain-side knows how difficult it is to distinguish a sheep, at some distance, from a mass of stone or rock.