With few exceptions, birds have a vocal organ, and are able to produce more or less variable sounds. The organ is, however, wanting in the running birds, such as the ostrich and the American vultures. The sounds produced are almost as varied as the different kinds of birds, and an expert ornithologist has little difficulty in identifying a great number of forms by their distinctive noises. That some chirp and others scream, that chattering describes the language of many and croaking that of others, that some boom and others bark, that the crows caw, and the laughing jackass laughs, that the mocking-bird imitates, and the parrot becomes able to articulate, and above all that the lark trills and the nightingale truly sings, are well-known illustrations of the variety of bird language. The weird cry of the curlew or whaup, the melancholy voice of the sea-mew, the gabble of ducks, the crowing of the cock, the soft cooing of the dove, the hoarse voice of the corncrake, the ecstatic melody of the bobolink, the cheerful notes of the blackbird, the educated music of the canary, are again a random selection of instances from an almost infinite medley. It is among the so-called perchers, songsters, or Insessores that we find song really developed, and that for most part in the males, and in highest degree at breeding time. Though the notes are not musically pure, many songs of birds have been expressed in musical notation, and every one is familiar with imitations in word form. Singing is an unbidden expression of emotional energy. It is most marked at the high tide of sexual emotion during the breeding season. It is best, sometimes solely, developed in the males, who use their powers to attract the females, and often vie with one another in so doing. In other cases the note is obviously used as language, expressing alarm and the like, for that some birds are able by voice to convey impressions to one another is indubitable. In so far as the song is an instrument and expression of sexual attraction, it fails to be included among those powers which have been strengthened and developed by sexual selection.