41 
on the migration of birds. 
The testes of the swift, which assume a singular oblong shape, 
somewhat exceed the cuckoo’s in bulk, though not so large 
as those of the wren. I have selected the wren as an example 
for this comparison, on account of its diminutive size. The 
testes of all those birds which are capable of producing young 
more than once in the breeding season, become tumid, as far 
as I have seen, in the same proportion as those of the wren. 
As there are many birds, which, if unmolested, produce 
but one nest of young ones in the course of the season, it may 
be asked, why nature should cause as great an enlargement 
of the testes in these, as those which breed more than once ; 
and why they should exceed in bulk those of the cuckoo or 
the swift ? The answer, I presume, is obvious. Should any 
ill accident befall the nestlings of the swift when advancing 
to maturity, the injury would be irreparable, the parent bird 
being destined to quit the country before another offspring 
could be reared. The cuckoo is in the same predicament ; 
but the wide dispersion of its young ones, (being placed 
singly in the nests of other birds), gives them such security 
as almost to preclude the possibility of their general destruc- 
tion.* But it is not so with those birds which make a longer 
stay ; should similar accidents befall them, they can repair 
their losses. Nature, as long as incubation could serve their 
weighed. For example : how very different must the weight of the owl be, which, 
in its nocturnal flights, had the luck to pick up a mole or two, compared with that 
which had met with opposite fortune ; or of the falcon, that had picked the bones 
of a leveret, or of the one that was killed with an empty stomach. 
* May not this be offered as another reason, why its eggs and young ones are 
entrusted to the fostering care of so great a variety of birds ? It could not have time, 
during its short stay, to rear so large a progeny ; and by no other means could it 
have placed its numerous brood so much out of the way of danger. 
MDCCCXXIV. G 
