522 Chimney and Window Swallows. 



This congregation seems only to arise from a sociable dis- 

 position ; yet it continues perhaps for six weeks, until the 

 party is reduced by migration to a very small number. 



My experience corroborates the remark, that the swallow 

 tribe disappears earliest in the warmest seasons. This seems 

 to be a mysterious circumstance, but may perhaps be explained 

 by reference to the bodily constitution of the birds. It is well 

 known that a certain temperature is necessary for hatching 

 the eggs of birds, and that, if this be exceeded, the death of 

 the embryo is the result. The temperature adapted to the 

 incubation of most tropical birds appears to be too great for 

 the eggs of the cuckoo and swallow. Under the direction, 

 therefore, of a guiding Providence, which has implanted 

 within them a sensibility of the due temperature, they pass 

 into the northern regions at the proper season. When this 

 great object has been accomplished, the body becomes fitted 

 for another service. Birds that remain with us through 

 the year require the warm temperature of autumn to enable 

 them to moult. A degree of feverish action is necessary to 

 this process ; and by raising it artificially and prematurely, 

 birdcatchers are known to accelerate the process of moulting. 

 This fever, as in the disorder of the same name in the human 

 body, is accompanied with a morbid sensibility, which renders 

 painful those impressions of the air which before were pleasing. 

 Instigated by these new sensations, they fly towards warmer 

 regions ; and, having there accomplished the natural process 

 of renewing the plumage, a change of constitution again leads 

 them to a cooler air. In many instances the marten has been 

 known to remain late, for the purpose of feeding the young 

 which have been slow to leave the nest ; and, as the moulting 

 fever does not commonly arise until the breeding constitution 

 has ended, this does not usually produce inconvenience : but 

 when the approach of the moulting constitution is felt before 

 the young are able to fly, parental fondness yields to febrile 

 excitement, and I have known them left to perish in the nest. 

 It is easy, then, to imagine how a warm summer, by inducing 

 the moulting fever early, causes their early departure ; while 

 a cold season delays them, by retarding it. 



At the season of departure, the number of our resident 

 birds lessens gradually ; and even those that go away together 

 soon separate. Neither do they proceed in haste : for those 

 which I have seen migrating seem to be employed in hawk- 

 ing for prey as usual. The following notes confirm the 

 remark that these birds pass off" in small companies, and that 

 those from more northern regions are often seen on the 

 passage after our own birds have departed : — 



