It will be remarked that all the birds mentioned live much upon the ground, obtaining the principal part of their food therefrom, and that therefore they have need of special protection. Incubation in every case takes place on the ground; and just as the imitation of the plumage of the female bird is perfect, so will the fact tell upon the survival of the species. There is no such need of protection for tree builders, as these, for the most part, are out of the way of predatory animals. The chaffinch is by far the most abundant bird of our fields and woods; and there is one good reason why it should be so. It invariably covers its nest on the outside with dead lichens, like to those of the trunk against which it is built. Against boys and other predatory creatures the device succeeds admirably, and the chaffinch as a species flourishes vigorously. The wren constructs her nest of moss, placing it upon a mossy background so as to present no sharp contrasts. Sometimes she interweaves one or two dead oak-leaves, so as to render the deception more deceitful; and, from the number of wrens which abound, she evidently succeeds. Starlings and sparrows and jackdaws, which build in holes at a considerable elevation, and have therefore less need of protection, hang out straws and sticks and bits of wool and feather as impudent advertisements. Wheatears and such birds as build in low walls cannot afford to do this, but instead build neat nests leaving no trace without. Several of our leaf-warblers drag dead leaves to the outside of their nests, and a hundred others employ like ingenuities.