Observations on Periodical Birds. 137 
when they appear, the very reverse of what 
ought to be the case if they become torpid, 
and of what is actually found to be so, with 
the sleeping animals of winter: indeed, it 
seems impossible that any animal should _be- 
come dormant, in a temperature superior to 
that which is required to revive it from its 
lethargic state: it is evident, therefore, that 
the birds in question must migrate. As there 
are, however, several other curious facts re- 
lating to the periodical birds, which throw 
great light on the subject of migration, and 
powerfully tend to confirm this opinion, I 
shall proceed to examine them. 
It isa very surprising circumstance, that 
several species of periodical summer birds 
almost constantly return to the same places 
in the same numbers; and there are sufficient 
reasons for believing that these birds are 
generally the same individuals. Four or five 
pairs of swallows, and about two pairs of 
redstarts and of flycatchers visit our family 
residence, in Crumpsall, every spring; and 
White in his Natural History of Selborne, 
p- 230, says, “ among the many singularities 
attending those amusing birds the swifts, I 
am now confirmed in the opinion that we have 
every year the same number of pairs: invari- 
ably:” and-again, “the number that I con- 
s 
