ON THE MIGRATION OF BIRDS. 15 
first two species usually rearing four or five and the 
last two young ones. What, then, becomes of this 
increase? If these young birds do not quit the 
country, why are they not seen in the ensuing 
spring? These are perplexing questions, questions 
which the advocates of torpidity will find it impos- 
sible to answer satisfactorily ; indeed they involve 
difficulties which can only be removed by admitting, 
what is undoubtedly the case, that these birds mi- 
grate, and that, being deserted by the old ones, and 
losing all recollection of the places where they were 
brought up, they are directed in their spring flight by 
fortuitous circumstances, and are thus diffused over a 
large portion of the globe. 
The highly interesting and important fact that 
several species of Periodical Summer Birds moult 
during the interval which elapses between their de- 
parture and reappearance, if generally known to orni- 
thologists, would, it is reasonable to suppose, have 
been frequently and strenuously urged as one of the 
most conclusive arguments which could be advanced 
in support of migration; but notices of this nature 
are extremely rare, as perhaps no part of the animal 
economy of the feathered tribes has been so greatly 
neglected by natural historians as their moulting. 
That Swallows, Swifts, Cuckoos, Redstarts, and 
Spotted Flycatchers moult during their absence 
scarcely admits of a doubt. I have cut feathers out 
of the wings and tails of Swallows, so that I could 
