18 ON THE MIGRATION OF BIRDS. 
are never found to retain their first feathers on their 
return in spring, they must moult in their absence ; 
and it is probable this may be the case with the 
Periodical Summer Birds generally * : but it would be 
unphilosophical to suppose that these birds, in a state 
of torpidity, when the animal functions are nearly 
suspended, can both throw off their old feathers and 
develope new ones; therefore they must seek those 
countries which supply a requisite degree of warmth, 
and a sufficient abundance of food, to enable them to 
change their feathers. It appears from the following 
passage, extracted from the ‘Manuel d’Ornithologie’ 
of M. ‘lemminck, second edition, p. 426, that, with 
regard to the Swallow and House-Martin, this distin- 
guished ornithologist had previously arrived at the 
same conclusion from a consideration of similar facts. 
“Je dois (he remarks) 4 M. Natterer de Vienne, 
Pobservation particuliérement intéressante, que les 
* Tn this attempt to prove that some of the Periodical Summer 
Birds 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 enumerated. 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 months 
in their full sexual plumage.” See the Supplement to the 
‘ Ornithological Dictionary.’ 
