Dr. Jenner on the Migration of Birds. 93 



are intrusted. Compared with quadrupeds, 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 leav- 

 ing us, may be looked upon as possessing power equal to the 

 old one in procuring food,' velocity 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 return- 

 ing calls of nature urge them to quit that countrv again to 

 which they are ?wjo 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 seasons 

 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 mi- 

 gration of winter birds of passage, and particularly of field- 

 fares 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. Catesbv, 

 Phil. Trans. No. 483. J 



The winter birds of passage, as they are commonly called, 

 begin to take their leave of us about the same time that the 

 spring migrators are taking wing to pay us their annual visit. 

 As the latter appear among us in gradual succession, so in 

 like manner the former disappear. They are both actuated 

 by the same impulse, the former in leaving, and the latter in 

 coming to this country, namely, the enlarged state of the testes 

 and ovarium. As soon as the stimulus becomes sufficiently 

 tilt, they quit their homes in quest of a country better suited 

 to their intended purpose than their own. 



That a want of food cannot be the inducement, must be 

 obvious to the slightest observer. When the redwing and 

 fieldfare quit this country, it abounds with that food which 

 they prefer to any other ; and at this time they are in the finest 

 condition; the redwings often enjoying their plenty by assem- 

 bling together on trees, and there uniting their feeble voices, 

 make no unpleasant song *. 



* The same thing happens through the winter, whenever the weather 

 tins long continued -.<> mild aa to allow them plenty of insert food. The 

 itarling (and some other birdi which have a short note and weak voice) 

 with it. companioni in the spring, and forms a similar concert. 



The 



