144 ON THE INSTINCTS OF BIRDS. 
built on the gratuitous assumption that several of the 
Periodical Summer Birds, as the Swallow, Spotted 
Flycatcher, Cuckoo, Goatsucker, &c., which feed almost 
entirely on insects, and consequently would not be 
able to procure a sufficient supply of nourishment 
in the winter months, have the property of passing 
the cold season in a state of torpidity—an hypothesis 
directly at variance with well-established facts. In- 
deed, how very defective and unsatisfactory the argu- 
ments advanced in support of the hybernating system 
are does not require insisting upon, as those who 
have considered the subject impartially must be well 
aware that they are almost wholly founded on the 
hearsay reports of ignorant and credulous persons. 
The history of the Cuckoo proves most incontro- 
vertibly that the desire to migrate, in that species, is 
instinctive, since nearly all the young ones brought 
up annually in the north of Europe quit it without 
receiving the least instruction that such a proceeding 
is requisite, and without any guide to direct them in 
their novel undertaking. But I forbear to dwell on 
the instincts of that extraordinary bird, partly on 
account of their being so very anomalous, but 
chiefly because I have already considered them at 
length*. The highly curious fact that the Swallow, 
House-Martin, Sand-Martin, and Puffin sometimes 
leave their last-hatched broods to die of hunger in the 
nest, in order to accompany their species in their 
* See observations on the Cuckoo, p. 49. 
