20 ON THE MIGRATION OF BIRDS. 
procure plenty of nourishment. Is it not rather occa- 
sioned by a disposition to moult, and the want of a 
suitable degree of warmth to enable them to change 
their feathers Our Domestic Fowls begin to moult 
in July, the hottest month in this latitude, and birds 
in a state of nature usually moult when they have 
done breeding ; if, therefore, the temperature of July 
is not sufficiently high to promote the moulting of the 
Periodical Summer Birds, Cuckoos, as they leave the 
care of their progeny to strangers, and, of course, are 
at liberty when they have deposited their eggs, should 
be the first birds which withdraw. Swifts also, having 
only two young ones to rear, should be the next birds 
which retire. The Periodical Warblers, and those birds 
which have five or six young ones, ought to quit in 
the next place; and Swallows and House-Martins, 
which have two broods in a season, ought to be the 
last that depart ; and this is always found to be the 
case; so that whether the departure of these birds be 
influenced by a disposition to moult or not, it seems 
to be regulated in a great measure by the cessation of 
their parental cares, and not by temperature solely. 
It will be difficult to produce any direct evidence 
of the migration of the Periodical Summer Birds until 
August ; and though deserted by its mate early in the month, it 
reared a second brood (the first having been destroyed) without 
assistance—a convincing proof that, however disagreeable it may 
be for Swifts to prolong their stay, they are not compelled to quit 
so eurly as they do by any difficulty in obtaining food. 
