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 in the American vultures. The
sounds produced are almost as varied as the different kinds of birds,
and an expert has little difficulty in identifying a great number of
forms by their distinctive noises. It is among the so-called perchers,
songsters, or Insessores, that we find song really developed and that
for the most part in the males, and in highest degree at breeding-time.