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 brood. 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 tenth of August,
and sometimes a few days sooner: and every straggler invariably
withdraws by the twentieth, 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 no ways influenced by any defect of heat; or, as
one might suppose, defect of food.
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