218 NATURAL HISTORY 
ers, since all other birds are known to moult soon 
after the season of laying their eggs. 
Swifts are very anomalous in many particulars, 
dissenting from all their congeners not only in the 
number of their young, but in having but one brood 
in a summer, whereas all the other British Azrun- 
dines have invariably two. It is past all doubt that 
swifts can have but one, 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 have 
but one brood in a summer, and only two at a 
time, and the other hirundines two, 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 10th of August, and some- 
times a few days sooner; and every straggler in. 
variably withdraws by the 20th; while their con- 
geners, all of them, stay till the beginning of Octo- 
ber, 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 more southerly parts of 
Andalusia, where they can be nowise influenced by 
any defect of heat, or, as one might suppose, de- 
fect 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 incidents 
in natural history that not only baffles our re- 
searches, but almost eludes our guesses ! 
