ON THE MIGRATION OF BIRDS. 13 
and of what is actually found to be so with the 
sleeping animals of winter; indeed, as torpidity does 
not appear to be induced in any British vertebrate 
animal by a degree of temperature superior to that 
which is required to revive it from its lethargic 
state, it is evident that the birds in question must 
migrate*. As there are, however, several other curi- 
ous facts relating to the Periodical Birds which 
throw great light on the subject of migration, and 
powerfully tend to confirm this opinion, I shall pro- 
ceed to examine them. 
It is a 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 Spotted 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 invariably ;” 
and again, “the number that I constantly find are 
eight pairs.” Now, as those birds usually make their 
* I have never been able to induce torpidity in the Cuckoo, or 
in birds of the Swallow tribe, by any experiments which I could 
devise ; though with animals of known torpid habits I have suc- 
ceeded without difficulty. 
