472 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 1918. 



pheasants and birds of paradise which, it has been assumed, was 

 the cause of the beautiful plumage of these birds. The female 

 choosing the best performer or the most highly colored male has 

 resulted through slight modification, generation after generation, 

 in these elaborate decorations. But we have, since Darwin, dis- 

 covered that the love dance or display is in some measure used 

 by many birds, often birds of dull color, like the English sparrow, 

 and they are still, in spite of the love dance, dressed in gray or 

 sober plumage. Howard, in his remarkable History of the British 

 Warblers, has shown " that these birds of sober hues perform dur- 

 ing moments of sexual exaltation, antics which in every way reflect 

 the display supposed to be peculiar to birds of brilliant plumage." 

 Savi's warbler also indulges in these antics even when feeding his 

 young. Furthermore, these dances are not confined to the period of 

 courtship. 



From whatever point of view we approach this subject the evidence 

 is so strong that we are compelled to look for our explanation in the 

 internal life of the bird rather than in any external, exciting cause. 

 Most of the theories thus far set forth have in them an element of 

 truth. If the purpose of song is excitation of the female to break 

 down her coyness, this very act may compel her to exercise an un- 

 conscious choice and thus sexual selection may exert a limiting and 

 directive force in the life of the bird. Even Hudson's theory that 

 the bird sings out of the abundance of its very being, joy and life, is 

 not to be ignored. But the question forces itself upon us, why does 

 the bird sing and dance to overcome the female coyness and what 

 gives the male more vitality than the female ? The answers to these 

 questions force us back into the inner life of the bird to seek our 

 answer in the essential difference between the sexes. 



So far as song, as well as other displays, in the mating season are concerned 

 they are due to the ripening of the sexual glands, from which, as Pycraft has 

 shown, hormones are set free, and, pervading the body, stimulate the nervous 

 system, and at the same time the secondary sexual characters — the antlers of 

 the stag, the mammary glands of the female, the " breeding plumage " of the 

 bird. When they are obviously secondary sexual characters, as in the case of 

 dull-colored birds, the result is the same, a state of physical exaltation ex- 

 pressed in " display." Males or females wherein these " hormones " are but 

 feebly developed, display and respond indifferently, and so cease to please the 

 opposite sex. As Mr. Howard has pointed out, in the case of the warblers, no 

 amount of display on the part of the male will avail until the female has at- 

 tained a like pitch of preparedness for the work of procreation. The courtship 

 of the ruffs and reeves, already referred to, affords another illustration. Here 

 it will be remembered the males for weeks spend laborious days in endeavor- 

 ing to gain some responsive sign from their prospective but phlegmatic mates, 

 yet without receiving the slightest sign of encouragement or recognition. As 

 soon, however, as the female has become " sexually ripe," as soon as the hor- 

 mones secreted by her generative glands have done their work, she herself in- 

 dulges in a species of nuptial dance, waltzing round her lord, and setting down 



