Observations on Periodical Birds. 139 
of proportion to the birds that retire; and 
this is uniformly the case. Now swallows 
and martins have frequently two broods ina 
summer; the first consisting of about five 
young ones, and the second of three, upon 
an average; and redstarts, flycatchers, and 
swifts, have one brood; the two first 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 en- 
suing spring? ‘These are perplexing ques- 
tions, questions which the advocates of tor- 
pidity will find it impossible to answer: 
indeed, they involve difficulties which can 
only be removed by admitting, what is un- 
doubtedly the case, that these birds migrate ; 
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 that elapses 
between their departure and re-appearance, 
if generally known to ornithologists, would, 
it is reasonable to suppose, have been fre- 
