Philosophical Transactions. 139 



nor his observations upon the incapability of dogs, ducks, and 

 other divers to remain iongj under water. 



The immediate cause of mio-ration is traced to those changes 

 which take place in the birds at the coming on of spring, and 

 which are subservient to the production of offspring ; it directs 

 them to seek a country where they can be for a while better ac- 

 commodated with succours for their infant brood, than in that 

 from which they depart ; and that nesting is the chief cause of their 

 errand is proved, by its occupying their attention from the day of 

 their arrival to that of their departure. The cuckoo is singularly 

 alert in this business, but as he deviates so widely from the com- 

 mon laws of the feathered society, our author selects the swift as 

 a better example. 



" 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 

 farther residence cannot be rendered unpleasant by any disaoree- 

 able 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 circumstance 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 natura- 

 list, the late Mr. White, of Selborne, 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 10th of August, and sometimes a few days 

 sooner, and every straggler invariably withdraws by the 20th, 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 November. 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 one of those inci- 

 dents in natural history that not only baffles our searches, but al- 

 most eludes our guesses !' Thus Mr. White. 



" Now, should the principle I have laid down be admitted, 

 namely, that these birds come here for scarcely any other purpose 

 than to produce an offsprin;.;- and retreat when the task is finished, 

 how easily will all circumstances be reconciled .? and how littlo 

 mysterious will those things appear which naturally seemed unac- 



