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Gregarity has a great effect on the frequency with which com- 
mon cries are uttered by birds, as the manners of our starlings, 
sparrows, finches, and hirundines clearly prove; but this may be 
counterbalanced by fear, as it is in the columbide, which 
survive by the agency of their timidity and great fecundity. 
Isolation has an equally apparent effect in limiting the cries of 
birds. The lonely bird may sing, but he never is garrulous. 
These facts greatly support a hypothesis that the voice, and 
especially in the production of call-notes, is generally employed 
as a means of inter-communication between birds. 
The influence of population and its relation to difficulty of 
obtaining food, is indicated even in the notes of birds; for 
these creatures, when not periodically destroyed by cold, as 
they are in temperate climates, are kept in check by enemies, 
or by lack of food; in either case they lose that abundant 
leisure or opportunity to sing, which is necessary to the 
development of all songs. 
The development of the eye in the search for food may, 
when not counterinfluenced by size, have disposed certain 
species towards the preferment of brilliancy of colouring rather 
than excellence of voice in their mates; and conversely, bifds 
dwelling in the obscurity of thick foliage would detect by ear 
rather than by eye the approach of their enemies; and they 
might thus acquire a delicacy of hearing which would be 
gratified by sweet songs. 
Darwin perceived that the language of some birds is 
artificial, in relation to certain enemies. 
The Hon. Daines Barrington educated or rather reared 
young linnets under various birds, for the purpose of discover- 
ing whether their songs were hereditary, or were acquired by 
imitiation ; he found that they were perpetuated through the 
latter agency. He mentioned many instances which, as he 
says, “seem to prove, very decisively that birds have not any 
minute ideas of the notes which are supposed to be peculiar 
to each species”’; and that the notes descend from parent to 
fledgling, because of “the nestlings attending only to the 
instruction of the parent bird, while it disregards the notes of 
