ON THE INSTINCTS OF BIRDS. 143 
though when domesticated it appears to have this 
faculty more highly developed than almost any other 
British bird. 
The congregating of gregarious birds, which takes 
place in autumn when they have finished breeding, is 
perhaps intended to promote their mutual security, as 
they are much less liable to be surprised by enemies 
when associated together in large numbers than they 
are when separate. What tends to strengthen this 
opinion is the fact that some species provide for the 
general safety by appointing sentinels to give notice 
of approaching danger. This social disposition, which 
(with the well-known exception of Rooks) usually 
continues no longer than the next pairing-season, 
seems, from the uniformity of the actions that result 
from it, to be of instinctive origin, though it certainly 
would be difficult to bring any direct proof that such 
is the case. 
In treating of the migration of birds, Dr. Darwin 
observes that as all species are capable of remaining 
throughout the year in those countries in which they 
were bred, any departure from them must be un- 
necessary, and therefore cannot be instinctive. This 
reasoning, however, is extremely fallacious, inasmuch 
as it restricts the operations of instinct solely to what 
is necessary ; whereas we have seen that the singing 
of birds and the practice of concealing their super- 
fluous food, though not absolutely indispensable, are, 
nevertheless, decidedly instinctive. It is, moreover, 
