To imagine that these beginnings
were intentionally made in order to be in the greater forwardness
for next spring, is allowing perhaps too much foresight and rerum
prudentia to a simple bird. May not the cause of these latebrae
being left unfinished arise from their meeting in those places with
strata too harsh, hard, and solid, for their purpose, which they
relinquish, and go to a fresh spot that works more freely ? Or may
they not in other places fall in with a soil as much too loose and
mouldering, liable to flounder, and threatening to overwhelm them
and their labours ?
One thing is remarkable -- that, after some years, the old holes are
forsaken and new ones bored; perhaps because the old habitations
grow foul and fetid from long use, or because they may so abound
with fleas as to become untenable. This species of swallow
moreover is strangely annoyed with fleas: and we have seen fleas,
bed-fleas (pulex irritans), swarming at the mouths of these holes,
like bees upon the stools of their hives.
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