92 Dr. Jenner on the Migration of Birds. 



now and then appear, that it overcomes even the obligation 

 of rearing their young when hatched late in the season, and 

 thev are sometimes left in a callow state to perish in their 

 nests. This premature departure, probably arises from a re- 

 verse of that stimulus which occasions the too early migra- 

 tion of the spring birds, as has been noticed in a former part 

 of this essay, namely, a change which lakes place in organi- 

 zation. 



One of the most singular occurrences in the history of mi- 

 gration, is the mode of departure of the young birds from the 

 country where they were produced. It may be conceived that 

 the bird which had once crossed the Atlantic, or any other 

 ocean, might have something impressed upon it that should 

 prove an inducement to its return ; but this cannot be an in- 

 citement to the young one. The identical bird, which but a 

 few weeks before burst from the shell, now unerringly finds, 

 without any apparent guide, a track that leads it safely to the 

 place of its destination, perhaps in many instances over the 

 widest oceans. 



It is well known, that those birds which incubate several 

 times in the course of one summer, forsake their first broods 

 when they no longer require their protection : and being now 

 alienated, they cannot, in their parents, find the guides that 

 conduct their course. As swallows and martins congregate* 

 prior to their departure from us, it may be said that their 

 young, though discarded, may mingle with the common flock, 

 and in this particular instance I am ready to admit that it is 

 probable they may do so ; but there are many migrating birds 

 that never either associate with swallows and martins, or join 

 together in flocks, as the nightingale, redstart, and indeed the 

 far greater number. As a striking proof that the parent bird 

 cannot possibly be the guide, in one instance at least, we may 

 point out the cuckoo, whose offspring finds a distant shore in 

 perfect safety, although it could never know the parent to 

 whom it was indebted for existence, and though 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 



* Swallows and martins congregate on the sunny sides of buildings for 

 the sake of warmth, and not, as it is generally supposed, to hold a kind of 

 consultation previous to their final departure. In the wet summer of 1821, 

 when the air was unusually chilled by the long continued rains, they were 

 observed to assemble, during some intervals of sunshine, for several succes- 

 sive mornings, ns early as the middle of July; and in the present year (1822) 

 I remarked the same on some mornings that were unseasonably cold about 

 the middle of August. 



are 



