50 
The late Dr. Jenner 
its existence in numberless instances must have taken place 
even after the departure of the parent. For the old cuckoos 
invariably leave us early in July, when many of their eggs 
are yet unhatched in the nests of those small birds to whose 
fostering care they are entrusted. Compared with quadru- 
peds, and some other animals, birds may be considered as 
acquiring the adult state at an early period, and the young 
bird, at the time of its leaving us, may be looked upon as 
possessing power equal to the old one in procuring food, ve- 
locity of flight, &c. The parent bird, from having lost that 
stimulus by the subsiding of the testes and ovaria, which 
urged it to incubation and detained it here, is now reduced to 
a condition similar to that of its offspring, both falling into 
the same habits, and remaining in the same state with respect 
to organization, until the returning calls of nature urge them 
to quit that country again to which they are now about to 
depart. 
II. 
Winter Birds of Passage. 
“We have, ’tis hoped, made it pretty evident that summer 
“ birds of passage come to and depart from us at certain sea- 
“ sons of the year, merely for the sake of a more agreeable 
“ degree of warmth, and a greater plenty of food ; both which 
“ advantages they procure by an alternate change of climate; 
“ but the migration of winter birds of passage, and particu- 
“ larly of fieldfares and redwings, is much more difficult to 
“ be accounted for, there being no such apparent necessity 
“ either on the score of food or climate, for their departure 
& from us.” 
Mr. Catesby, Phil. Trans. No. 483- 
