Page:The Zoologist, 4th series, vol 3 (1899).djvu/423

Rh colour, I felt sure that they must be covered, and, gazing still more attentively, all at once, by an optical delusion as it seemed, rather than by the passing away of one, the piece of fir-bark became the bird. The broad flat head, from which the short beak hardly projects noticeably, presents no special outline for the eye to seize on, but is all in one line with the body. It looks just like the blunt rounded end of a stump or piece of fir-bark, whilst the dark brown lines and mottlings of the feathers not only blend with and fade into the surroundings, but have in themselves, at a little distance, a great resemblance to the flaked surface of the bark, the lighter feathers exactly mimicking those patches where some of the layers have been more newly flaked off. This would only be of special advantage to the bird when, as in the present instance, it had laid its eggs amidst pieces of fir-bark, and, did it invariably do so, a special protective resemblance might perhaps be admitted. This, however, is not the case. It lays them also under beeches or elsewhere where no fir-bark is to be found. Unless therefore it could be shown that a large majority of Nightjars lay their eggs in the neighbourhood of fir-trees, the theory of a special resemblance due to the action of natural selection must be given up, as I believe it ought to be in other apparent instances. No doubt when the objects adjacent are different the sitting bird may often appear to have a special resemblance to one or other of them; but as, owing to its habits, such objects would be mostly of the same general description, the bird's colouring may have been made generally protective in relation to its incubatory habits. The Nightjar lays on the ground, and one of the birds sits on the eggs without leaving them the entire day. Day, however, is night to the Nightjar, which not only sits on its eggs, but sleeps, or a least dozes, on them as well. It is therefore much exposed during this period, and would be liable to be taken unawares without some protection, and such protection it has by virtue of its plumage and its habit of sitting very close. Drowziness may in this case have meant security both to bird and eggs, for the most sleepy birds would, by keeping still, least endanger their young at all stages.

The two birds that I am watching have laid their eggs in the midst of pieces of fir-bark of various sizes lying on a sandy soil,