Observations on Periodical Birds. 143 
functions are nearly suspended, can both 
throw off their old feathers, and put out new 
ones; therefore, they must seek those coun- 
tries which supply a requisite degree of 
warmth, and a sufficient abundance of food, 
to enable them to change their feathers. Old 
cuckoos leave us late in June or early in July, 
when the temperature is approaching the 
maximum for the year; and swifts retire 
about the middle of August, when. the tem- 
perature though receding from the maximum, 
is still very high.. To what cause, then, shall 
we attribute the early retreat of these birds? 
certainly not to a deficiency of food, as young 
cuckoos are frequently found to remain up- 
wards of a month after the old birds have 
left ; and swifts are occasionally seen, long 
after the great, body of their congeners has 
withdrawn ;* and yet these birds procure 
moult during their absence, I have purposely confined my remarks to 
such species as are well known, and easily observed. The red-backed 
shrike, whinchat, and‘some others, with whose habits and economy we 
are less familiar, might, however, be added to those already enume- 
rated. In my opinion respecting the moulting of the red-backed shrike, 
I am supported by Montagu ; who affirms, “ that all the young, when 
they leave us in the month of September, very much resemble the 
adult female ; and the whole return to us again in about six monthis, 
in their full sexual plumage.” See the Supplement to his Ornithological 
Dictionary. 
* In the year 1815, I sawa swift in the township of Crumpsall, on 
the 20th of October; and the same bird was seen again on the 25th ; 
which is more than two months beyond the time at which these birds 
