hee" 
j THE NATURAL HISTORY OF SELBORNE. -  J6I 
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 broods. We may here 
remark, that, as swifts breed but once in a summer, and only 
two at a time, and the other A/irundines 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 
roth of August, and sometimes a few days sooner; and every 
straggler invariably withdraws by the 2oth, while their con- 
geners, 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 in no ways influenced by any defect of heat ; 
or, as one might suppose, failure of food. Are they regulated 
in their motions with us by a defect of food, or by a pro- 
pensity 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 searches, but almost eludes 
our guesses ! 
These Hirundines never perch on trees or roofs, and so never 
congregate with their congeners. They are fearless while 
haunting their nesting-places, and are not to be scared with a 
gun; and are often beaten down with poles and cudgels as they 
stoop to go under the eaves. Swifts are much infested with 
those pests to the genus called bird-ticks; and often wriggle 
and scratch themselves in their flight to get rid of that clinging 
annoyance. 
