68 Proceedings of Philosophical Societies. [Jan. 



feed ; and not because they have arisen from the waters. Their 

 usual food, like that of swifts and martins, is a species of sca- 

 rabseus, 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 outthe skeletons, 

 and laid in the nest again. 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 is remarked, cannot be directed in their 

 migratory flights by the parents, but must be guided by some 

 unknown principle : if it be admitted in the case of swifts, 

 martins, and other birds associating together in flocks, that the 

 young may be directed by the motions of their fellows, yet this 

 cannot be the case with the nightingales ; nor with the cuckoos, 

 who, though reared in the nests of many different birds, are re- 

 gular migrators. The parent cuckoo has left the country before 

 its young are reared, always departing early in July. 



Dr. Jenner next gives some particulars relative to the enlarge- 

 ment of the testes and ovaria in birds, supplementary to those 

 which have been pointed out by Mr. John Hunter. In those birds 

 who pair but for a short time the testes are small, while in those 

 with whom the connubial compact is of long continuance, they 

 are large. In the cuckoo, a polygamist, and who continues 

 with the female but for a very short time, the testes are of the 

 size of a vetch only ; but in the wren, whose attachment to his 

 mate extends from spring to autumn, they are equal to a pea in 

 magnitude ; thus much larger in the latter than in the former, in 

 proportion to the size of the bird. A continued supply of gene- 

 rative power is required in birds who pair for a long time, in 

 case the brood should be destroyed — but in those like the cuckoo 

 this provision is unnecessary. 



The whiter birds of passage leave this country for precisely 

 the same reason that impels the spring migrators to come hither; 

 some of them, as the wild-duck and the wood-pigeon, which 

 occasionally build here, are irregular in their migration; the 

 most regular are the red-wing and the field-fare, of whose 

 building in this country Dr. Jenner never met with an instance. 

 The food of the former, he observes, is not haws, or the fruit of 

 the white thorn, as has been stated, but worms and insects, 

 which they gather from the ground, feeding in flocks ; Dr. J. 

 had seen them dying of famine when haws were abundant. A 

 gentleman saw a flock of field-fares on the day before the thaw- 

 ing of the great frost of 1794, and they seemed as wild and vi- 

 gorous as if in season ; he shot one, which Dr. Jenner examined, 

 and found to be in excellent condition, but there was no food in 

 the stomach, and the last which the animal had eaten was di- 



