with heavy showers, they dislike, and on such
days withdraw, and are scarce ever seen.

There is a circumstance respecting the colour of swifts which seems not
to be unworthy of 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 pursue 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 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.

But in nothing are swifts more singular than in their early retreat. They
retire, as to the main body of them, by the 10th 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 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 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 searches, but almost eludes our
guesses!

These hirundines never perch on trees o