Evolution and Adaptation

by Thomas Hunt Morgan

In the group of birds we find some of the most striking cases of secondary sexual differences. The spurs, combs, wattles, horns, air-filled sacs, topknots, feathers with naked shafts, plumes, and greatly elongated feathers are all secondary sexual characters. The songs of the males, the rattling together of the quills of the peacock, the drumming of the grouse, and the booming sounds made by the night jars while on the wing, are further examples of secondary sexual differences. The odor of the male of the Australian musk duck is also put in the same category.

The pugnacity of many male birds is well known, and it is imagined that one of the results of the competition of the individuals of the same sex with each other has led to the development of the organs of defence and offence. The males that have been successful in these battles are then supposed to mate with the best females. In this way those secondary sexual differences, connected with the encounters of the males, are supposed to have been formed. Darwin states in this connection:—

“Even with the most pugnacious species it is probable that the pairing does not depend exclusively on the mere strength and courage of the male; for such males are generally decorated with various ornaments, which often become more brilliant during the breeding season, and which are sedulously displayed before the females. The males also endeavor to charm or excite their mates by love-notes, songs, and antics; and the courtship is, in many instances, a prolonged affair. Hence it is not probable that the females are indifferent to the charms of the opposite sex, or that they are invariably compelled to yield to the victorious males.”

Thus a double process of selection is imagined to take place; one, the outcome of a competition of the males with each other, and the other, through a choice of the more successful males by the females, the more beautiful being supposed to be chosen.

It may be well not to lose sight of the fact that unless the selection is severe in each generation, its good effects will be lost, as has been stated in connection with the theory of natural selection. Still more important is the consideration that unless the same variations appear at the same time, in many of the surviving males, the results will be lost through crossing. These statements will show that the difficulties of the theory are by no means small, and when we are asked to believe further that another process still has been superimposed on this one, namely, the selection of the more beautiful males by the females, we can appreciate how great are the difficulties that must be overcome in order that the process may be carried out.

The love-antics and dances of male birds at the breeding season furnish many curious data. The phenomena are imagined by Darwin to be connected with sexual selection, for in the dances the males are supposed to exhibit their ornaments to the females who are imagined to choose the suitor that is most to their taste.

Hudson, who has studied the habits of birds in the field, asks some very pertinent questions in connection with their performances of different kinds. “What relation that we can see or imagine to the passion of love and the business of courtship have these dancing and vocal performances in nine cases out of ten? In such cases, for instance, as that of the scissortail tyrant-bird, and its pyrotechnic displays, when a number of couples leave their nests containing eggs and young to join in a wild aërial dance; the mad exhibitions of ypecahas and ibises and the jacana’s beautiful exhibition of grouped wings; the triplet dances of the spur-winged lapwing, to perform which two birds already mated are compelled to call in a third bird to complete the set; the harmonious duets of the oven-birds and the duets and choruses of nearly all the wood-hewers, and the wing-slapping aërial displays of the whistling widgeons,—will it be seriously contended that the female of this species makes choice of the male able to administer the most vigorous and artistic slaps?”

“The believer in the theory would put all these cases lightly aside to cite the case of the male cow-bird practising antics before the female, and drawing a wide circle of melody around her, etc.... And this was in substance what Darwin did.” “How unfair the argument is based on these carefully selected cases gathered from all regions of the globe and often not properly reported is seen when we turn to the book of nature and closely consider the habits and actions of all the species inhabiting any one district.” Hudson concludes that he is convinced that any one who will note the actions of animals for himself will reach the conviction, that “conscious sexual selection on the part of the female is not the cause of music and dancing performances in birds, nor of the brighter colors and ornaments that distinguish the male.”

The differences in color in the sexes of birds are classified by Darwin as follows: (1) when the males are ornamented exclusively or in a much higher degree than the females; (2) when both sexes are highly ornamented; (3) when the female is more brightly colored. A few examples of each sort may be chosen for illustration.

“In regard to color, hardly anything need here be said, for every one knows how splendid are the tints of many birds, and how harmoniously they are combined. The colors are often metallic and iridescent. Circular spots are sometimes surrounded by one or more differently shaded zones, and are thus converted into ocelli. Nor need much be said on the wonderful difference between the sexes of many birds. The common peacock offers a striking instance. Female birds of paradise are obscurely colored and destitute of all ornaments, whilst the males are probably the most highly decorated of all birds, and in so many different ways, that they must be seen to be appreciated. The elongated and golden-orange plumes which spring from beneath the wings of the Paradisea apoda, when vertically erected and made to vibrate, are described as forming a sort of halo, in the centre of which the head ‘looks like a little emerald sun, with its rays formed by the two plumes.’”