A disease which may be called the _pairing fever_ must not be forgotten
here. House birds are usually attacked with it in May, a time when the
inclination to pair is greatest. They cease to sing, become sorrowful
and thin, ruffle their feathers, and die. This fever generally first
seizes those which are confined in cages: it appears to arise from their
way of life, which is too uniform and wearying. I cured several by
merely placing them in the window, where they are soon so much refreshed
that they forget their grief, their desire for liberty or for pairing,
and resume their liveliness and song.
I have observed that a single female in the room is sufficient to cause
this disease to all the males of the same family, though of different
species. Removing the female will cure them directly. The males and
females at this season must be separated, so that they cannot see or
hear one another. This perhaps is the reason that a male, when put in
the window, is soon cured.