MIGRATION. 87 



morning : at first there was a vast fog ; but, by the time that 

 I was got seven or eight miles from home towards the coast, 

 the sun broke out into a delicate warm day. "We were then 

 on a large heath, or common, and I could discern, as the 

 mist began to break away, great numbers of swallows 

 (Jiirundines rusticfs) clustering on the stunted shrubs and 

 bushes, as if they had roosted there all night. As soon 

 as the air became clear and pleasant, they all were on the 

 wing at once ; and, by a placid and easy flight, proceeded on 

 southward, towards the sea : after this I did not see any 

 more flocks, only now and then a straggler. 



I cannot agree with those persons who assert, that the 

 swallow kind disappear some and some, gradually, as they 

 come ; for the bulk of them seem to withdraw at once ; only 

 some stragglers stay behind a long while, and do never, 

 there is the greatest reason to believe, leave this island. 

 SwalloAvs seem to lay themselves up, and to come forth in a 

 warm day, as bats do continually of a warm evening, after 

 they have disappeared for weeks. For a very respectable 

 gentleman assured me that, as he was walking with some 

 friends, under Merton-wall on a remarkably hot noon, 

 either in the last week in December, or the first week in 

 January, he espied three or four swallows huddled together 

 on the moulding of one of the windows of that college. 

 I have frequently remarked that swallows are seen later at 

 Oxford than elsewhere : is it owing to the vast massy build- 

 ings of that place, to the many waters round it, or to what 

 else? 



they reach their final quarters ; and during this time having their supply of 

 suitable food daily augmented. 



The causes influencing the migration of birds, appear more difficult to solve 

 than the possibility of the execution of it. They seem to be influenced by 

 an innate law, which we do not, and cannot, comprehend, though in some 

 measure dependent on the want of food or climate congenial to the system of 

 each, and which acts almost without the will of the individual. Neither this, 

 however, nor the duties incumbent on incubation, can be the only exciting 

 causes, as we may judge by the partial migrations of some to different parts 

 of the same country, where food and the conveniences for breeding are alike ; 

 by the partial migration only, of a species from one country to another, 

 differing decidedly in temperature, and where the visiting species thrives 

 equally with the resident one ; and by the males of some species migrating, 

 while the females remain. W. J. 



