100 Dr. .Tenner on the Migration of' Birds. 



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 destruction *. 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 purposes, 

 would keep an accumulation of the proper powers in store, 

 which, in the case of the cuckoo and swift, would be entirely 

 useless. 



Whether there be a regular gradation in the size of the 

 testes (that of the bird itself being considered) throughout the 

 whole race, in proportion to the time taken up in pairing, I 

 cannot determine, not having had an opportunity of subjecting 

 the matter to a full investigation. However, I thought the 

 fact already shown of sufficient importance in natural history, 

 to be worthy of communication, as it forms a kind of sequel 

 to Mr. Hunter's paper on the subject. 



With due deference to the late Dr. Darwin, I am inclined 

 to think that the opinion he set forth respecting the pairing of 

 cuckoos, was taken up hastily, and that the birds which his 

 friend saw were not cuckoos feeding their nestlings, but goat- 

 suckers, whose mode of nesting corresponds with the relation 

 given, and whose appearance might be mistaken for them by 

 one not perfectly conversant with the plumage and the general 



* May not this be offered as another reason why its eggs and young 

 ones are intrusted to the fostering care of so great a variety of birds? It 

 could not have time, during its short slay, to rear so large a progeny ; and 

 l>y no other means could it have placed its numerous brood so much out 

 of the way of danger. 



appearance 



