and on such days withdraw, and are scarce ever 

 seen. 



Ttiere is a circumstance respecting the colour of 

 swifts which seems not to be unworthy of our atten- 

 tion. 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 re- 

 turn 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 hirundincs 

 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 

 broods. 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. 



37 



