NATURAL HISTORY OF SELBORNE. 
133 
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 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 occa- 
sionally 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 propensity to moulting, or by a disposition to rest 
after so rapid a life, or by what 1 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 con- 
gregate 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 hippoboscce 
hirundinis ; and often wriggle and scratch themselves in their flight to 
get rid of that clinging annoyance. 
Swifts are no songsters, and have only one harsh screaming note ; 
yet there are ears to which it is not displeasing, from an agreeable 
association of ideas, since that note never occurs but in the most lovely 
summer weather. 
They never can settle on the ground but through accident ; and when 
down, can hardly rise, on account of the shortness of their legs and 
the length of their 
wings ; neither can . . _^^^Mpii^^. 
The particular formation of the foot discriminates the swift from all 
the British hirundines ; and indeed from all other known birds, the 
they walk, but only 
crawl ; but they 
have a strong 
grasp with their 
feet, by which 
they cling to walls. 
Their bodies being 
flat they can enter 
a very narrow 
crevice ; and where 
they cannot pass 
on their bellies 
they will turn up 
edgewise. 
WHItE-BELLIED SWIFT, 
