Dr. Jenner on the Migration of Birds. 101 



appearance of cuckoos when on the wing. Is it probable that 

 the cuckoo, which is invariably a polygamist, and never pairs, 

 nests, or incubates in this part of the island, should fall into 

 opposite habits in another part? 



To recapitulate the substance of my observations. I have 

 first adduced some arguments in support of migration, the 

 fact itself not being generally admitted by naturalists of cele- 

 bi'ity, and also against the hypothesis of a state of torpor, or 

 what may be termed the hybernating system. I have repre- 

 sented that the swallow tribe, and many other birds that ab- 

 sent themselves at stated periods, return annually to the same 

 spot to build their nests ; and at the same time that any in- 

 ference drawn from this fact in support of a state of torpor, 

 would be fallacious upon physiological principles. That cer- 

 tain periodical changes of the testes and ovaria, are the in- 

 citing causes of migration. I have stated many facts, hitherto, 

 I believe, unnoticed, chiefly with respect to the cause which 

 excites the migrating bird, at certain seasons of the year, 

 to quit one country for another, viz. the enlargement of the 

 testes in the male, and ovaria in the female, and the need of a 

 country where they can for a while be better accommodated 

 with succours for their infant brood, than in that from which 

 they depart. It is also attempted to be shown that their de- 

 parture from this country is not in consequence of any dis- 

 agreeable change in the temperature of the air, or from a 

 scarcity of their common food, but the result of the accom- 

 plishment of their errand, i. e. the incubation, and rearing of 

 their young, and the detumescence of the testes and ovaria. 

 That successive arrivals of migrating birds are attributable to 

 the progressive development of the generative system in the 

 male and female ; that progressive developments are wise pro- 

 visions of nature ; that premature arrivals and departures are 

 frequently to be accounted for on the same principle ; that the 

 departure of the spring migrators is owing to a change in the 

 testes and ovaria, the very opposite to that which took place 

 in the spring; that the departure of the young birds is not 

 guided by the parent, but the result of an unknown principle. 



In the second part of this paper, some observations are 

 made on the winter birds of passage; that they quit their 

 homes (this country) in spring, in quest of a country better 

 •suited to their intended purpose than their own; that they 

 are actuated by the same impulse in quitting this country that 

 causes the spring birds to come to it, and that want of food 

 cannot be the inducement; that the emigration of the winter 

 birds is less complete than that of the others (the spring mi- 

 grators); that BOme species breed here, especially the wild- 

 duck 



