Proceedings of Learned Societies. 127 



tended to afterwards, but at the expiration of seven years from 

 their original capture, one of these marked Swifts was brought in 

 by a cat. 



Dr. Jenner next proceeded to state, as the cause of the migra- 

 tion of birds, that the tumid and enlarged state of the testes in the 

 male, and of theovaria in the female, at the season of their depar- 

 ture, prompt the animals to seek those countries where they can 

 obtain proper succours for their offspring ; — that, in fact, the nest- 

 lings are the objects of this provision. The parent birds leave the 

 countries they migrate from at a time when their own wants are 

 completely supplied ; and they remain in those to which they mi- 

 grate, no longer than suffices for the rearing of their young. Thus 

 the Swifts arrive in this country about the 5th or 6th of April, and 

 depart hence about the 10th of August. — Dr. Jenner here observed, 

 as a remarkable circumstance, that Ray, who attributed the mi- 

 gration of fishes to its true cause, that of seeking proper situations 

 for spawning, overlooked the corresponding impulse as actuating 

 birds. The Martins leave this country successively, some conti- 

 nuing to rear a brood much later than others : many of these birds 

 roost in the walls of Berkeley Castle ; and Dr. Jenner found, by 

 dissecting a number, taken at the same time, that the ovaria of 

 the females were in a variety of states ; in some the eggs being no 

 bigger than hemp seed, while in others they were as large as peas ; 

 the testes of the males exhibiting analogous degrees of tumidity. 



Swallows are seen flying over pools and waters in spring, in 

 search of the gnats on which they are then obliged to feed ; and 

 not because they have arisen from the waters. Their usual food, 

 like that of Swifts and Martins, is a species of scarabaeus, as the 

 author ascertained by dissection. 



Birds that rear several broods in the season, frequently leave 

 the last brood to perish ; thus a pair of Swifts that had brought up 

 three broods in one nest left the fourth to perish ; and the mother 

 came back in the following year, threw out the skeletons, and laid 

 in the nest a:rain. Many nests of late birds, of various species, are 

 deserted in this manner by the parent animals ; but the latter thus 

 leave the country when it abounds with their own food. 



The young birds, it wa» remarked, cannot be directed in their 



