y that they are
related to the "territory," but that they contribute not a little to the
successful attainment of reproduction.

The vocal productions are infinite in variety and combination. At the
one extreme we have songs composed of a single note repeated slowly or
rapidly as the case may be, whilst at the other we have the complex
productions of the Warblers; and between these two extremes, notes and
phrases are combined and recombined in ways innumerable. And just as
there is a rich variety of combination, so there is a very wide
variation in the purity and character of the notes--some are harsh,
others melodious, some flute-like, others more of a whistle, and others
again such as can only be likened to the notes of a stringed instrument.
Hence in variety of phrase combination added to variety in the character
of the note, there is a possibility of infinite modes of expression.

If, in the latter part of May, we take up a position at dawn in some
osier bed, we listen to songs which have reached a high degree of
specialisation, songs, moreover, which appeal to us on account of their
beauty; if, on the other hand, we climb down the face of the sea cliff,
we hear an entirely different class of songs--harsh, guttural, weird,
monotonous sounds, which, appeal to us though they may, lack the music
of the voices in the osier bed. And just as, in the osier bed, we can
recognise each species by its voice, so we can distinguish the "cackle"
of the Fulmar, the "croak" of the Guillemot, or the "grunt" of the Shag.
In the osier bed, however, there is considerable variation in the song
of different individuals of the same species, so much so that we can
recognise this one from that; whereas on the cliff we cannot distinguish
between the voices of different individuals. And the more highly
developed the song, the greater the range of variation appears to be;
but notwithstanding this--notwithstanding the fact that the pitch may
differ, the phrase combination may differ, and the timbre may
differ--the song remains nevertheless specific. So that the two
principal features of "song," broadly speaking, are "diversity" and
"specific character."

In contrast with the call-notes, the majority of which can be heard at
all times of the year, the song is restricted as a rule to one season,
and that one the season of reproduction. It is true, of course, that
some birds sing during the autumn, and, if the climatic conditions are
favourable