Dr. Jenner on the Migration of Birds. 57 



vincing proof that nesting is the chief cause of their errand 

 here, this and its natural consequences occupy their attention 

 from the time of their coming to the day of their departure. 

 This is illustrated by the dispatch which some of them make 

 in performing the object of their mission. The cuckoo finishes 

 this business in a shorter space of time than any other bird ; 

 but as he deviates so widely from the common laws of the fea- 

 thered society, I shall select the swift as a better example for 

 pointing out the fact. The swift shows himself here about the 

 beginning of May (sometimes a few stragglers appear earlier), 

 and by the beginning of August he has completely reared his 

 young ones, which seldom consist of more than two. At once 

 the old birds and their family take their leave and are seen 

 no more for that season. Now his further residence cannot 

 be rendered unpleasant by any disagreeable change in the 

 temperature of the air, or from a scarcity of his common food, 

 which at this time abounds in the greatest plenty. This cir- 

 cumstance of the early departure of the swift, without a more 

 apparent cause, seems to have excited much astonishment 

 and perplexity in the mind of that attentive and ingenious na- 

 turalist, the late Mr. White of Sel borne. Speaking of the 

 swift (Letter XXI. page 184), he says, " But in nothing are 

 swifts more singular than in their early retreat. They retire, 

 as to the main body of them, by the tenth of August, and 

 sometimes a few days sooner ; and every straggler invariably 

 withdraws by the twentieth, while their congeners all of them 

 stay till the beginning of October, many of them all through 

 that month, and some occasionally to the beginning of Novem- 

 ber. This early retreat is mysterious and wonderful, since 

 that time is often the sweetest season of the year. But, what 

 is more extraordinary, they begin to retire still earlier in the 

 most southerly parts of Andalusia, where they can be no ways 

 influenced by any defect of heat, or, as one might suppose, 

 defect of food. Are they regulated in their motions with us 

 by a failure of food, or by a propensity to moulting, or by a 

 disposition to rest after so rapid a life, or by what ? This is 



complete, when another fell a victim to the gun. Thus the gentleman went 

 on destroying one of them daily until he had killed seven; but all to no 

 purpose — the remaining magpie soon found another mate. The nest was 

 finished, and young ones were produced, which were suffered to fly. This 

 is an extraordinary fact. — It seems to show that nature lias a reserve of 

 birds in an unconnected state, immediately ready to repair losses. Were the 

 whole to pair at once, the circumjacent country might be insufficient to 

 furnish food for the immense number of young ones that must burst forth at 

 the same time. 



Vol. 64. No. S15. July IR2k H one 



