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There is a circumstance respecting the colour of swifts, 

 which seems not to be unworthy our attention. When 

 they arrive in the spring they are all over of a glossy, dark 

 soot-colour, except their chins, which are white ; but, by 

 being all day long in the sun and air, they become quite 

 weather-beaten and bleached before they depart, and yet 

 they return glossy again in the spring. Now, if they pur- 

 sue the sun into lower latitudes, as some suppose, in order 

 to enjoy a perpetual summer, why do they not return 

 bleached ? Do they not rather perhaps retire to rest for 

 a season, and at that juncture moult and change their 

 feathers, since all other birds are known to moult soon 

 after the season of breeding. 



Swifts are very anomalous in many particulars, dissenting 

 from all their congeners not only in the number of their 

 young, but in breeding but once in a summer ; whereas all 

 the other British hirundines breed invariably twice. It is 

 past all doubt that swifts can breed but once, since they 

 withdraw in a short time after the flight of their young, 

 and some time before their congeners bring out their 

 second brood. We may here remark, that, as swifts breed 

 but once in a summer, and only two at a time, and the 

 other hirundines twice, the latter, who lay from four to six 

 eggs, increase at an average five times as fast as the former. 



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 November. This 

 early retreat is mysterious and wonderful, since that time 

 is often the sweetest season in 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 ? 



