[Footnote 13: I instance only what I have actually seen, and go no farther.]
An incident which I have recorded elsewhere seems to me to bear upon the foregoing remarks.[14] Here a stone-curlew that had been sitting quietly for some time rose and uttered some shrill cries, in obedience to which another came running up, and after the two, standing close together, had each assumed a remarkable and precisely similar posture, the nuptial rite was performed. Were it not that, even by the witnessing of this last, it is not always possible to differentiate the sexes of birds, I could say with certainty that it was the female stone-curlew, in this instance, that called up the male; but the very striking attitude which the birds assumed, and which, if it was not a sexual display, it is difficult to know what to call it, was identical in both. Again, in the case of a pair of crested grebes that I watched during two successive springs everything (and there was once something very striking) in the nature of an antic or display was indulged in equally by the male and female. Peewits, also, behave during the nuptial season in a very marked manner, both whilst flying and upon the ground, and as far as I can make out--though I will not here speak with certainty--the conduct of both sexes is the same throughout.
[Footnote 14: _Bird Watching_, pp. 18-19.]
The nuptial cries or notes of birds are a chief way in which the one sex, on the theory of sexual selection, endeavours to render itself pleasing to the other. When these charm our own ears to an extent which we think deserving of the name of song, it is usually the male alone that utters them, those uttered by the female not rising to the height of such a definition. To how great an extent this law prevails I have not the knowledge to say, but it is not universal. The female canary, robin, lark, and bullfinch all sing, especially when widowed, though their song is not equal to that of the male, whilst in the red oven-bird of Argentina both sexes frequently join one another for the express purpose of singing a duet. Surely in this last case, especially, if it be assumed that the song of the male is uttered with the purpose of pleasing the female, or has that effect, the converse ought also to be assumed: and if so, why should not the hens, as well as the cocks, be sometimes chosen for their song?