Face page 156.]
But Sexual selection does not begin, and end, with the evolution of frills and furbelows. “Behaviour” counts for more than is generally supposed. This is as specific as “structure,” that is to say, it is as constant for each species as is its coloration, and it is also as variable. That Evolution may be determined by variation in behaviour, no less than through structural variations, is a possibility which has received but little consideration at the hands of students of Evolution.
The singular history of the Australian Bower-birds lends additional support to this view, and at the same time provides an additional argument against the generally accepted opinion that bright colours have been evolved by reason of the preference shown by the females for the most vividly coloured of their suitors. For while the males affect all the tricks and turns which are the common accompaniment of courtship, they, in addition, introduce very extraordinary features in the shape of “bowers” cunningly constructed and often gaily decorated, as will be seen presently. Eight of the total number of species of this group exhibit this behaviour, and while they differ very conspicuously in coloration among themselves, they agree very closely in the type of the bower they build. If the coloration is determined by the female, then in this they display very different standards, and if they do select, each according to the standard of the species, then we must suppose that they also must exercise a choice in regard to the character of the bower, the favoured male being the best builder. But why, in this case, is there not as much diversity in the form of the bowers as in the coloration of the feathers? A survey of the facts will perhaps make this point clear.
One of the best known of these bowers is that of the Satin Bower-bird (Ptilonorhynchus violaceus). On either side of a platform of small twigs a fence of similar twigs is reared, sloping inwards to form a more or less complete tunnel. At the entrance to this is placed a platform of sticks, which is strewn with a miscellaneous assortment of brightly coloured feathers, bleached bones, and occasionally flowers. The work of construction is almost entirely performed by the male: it is indeed a little curious, having regard to the circumstances, that the female should bear any share in its construction at all.
Really this is a more wonderful piece of architecture than would appear from the mere description of its main features: for it represents psychical activities which are difficult to fathom. It does not take the place of display, but is an extension of this. During his amorous moments the cock becomes greatly excited, chasing his mate in and out of the bower, carrying the while, in his beak, a brightly coloured feather or a leaf.
At the same time he sets all his feathers on end and every now and then drops first one wing, then the other, accompanying these actions with curious whistling notes and pretences of picking up food.
The Regent-bird (Sericulus melinus) differs conspicuously from the Satin Bower-bird, for while this is of a uniform, deep, metallic steel-blue, the Regent-bird is jet black, with a golden yellow crown and hind-neck and a great blaze of golden yellow on the wing. Yet the bowers of the two species—which belong to different genera—are practically identical, save that brightly coloured berries are used more frequently by the Regent-bird.
Plate 29.
Photo by W. P. Dando.