[Illustration: _The Hunting Flight._]

Ah, yes, there may be some reason for the belief that swallows bring
good luck to men. I once heard of a farmer who said he didn't dare
disturb these birds because of a superstition that, if he did, his cows
wouldn't give so much milk. Well, maybe they wouldn't if all the flies
a colony of swallows could catch were alive to pester his herd; for the
happier and more comfortable these animals are, the healthier they are
and the more milk they give.

The hunting flights of Eve and Petro and their comrades lasted about
fifteen minutes each time they took a recess from their building.

After two days the nest was big enough, so that there was room for both
swallows to build at once; and after that, Petro didn't have to fly
around with his mouth full of plaster waiting for Eve to go if he
chanced to come before she was through. They always chatted a bit and
then went on with their work, placing their plaster carefully and
bunting it smooth on the inside, modeling with clay a house as well
suited to their needs as is the concrete mansion a human architect makes
suited to the needs of man.

And if you think it is a simple matter to make a nest of clay, just go
to the wisest architect you know and ask him these questions. How many
hodfuls of clay, each holding as much as half a thimble, would it take
to build the wall of a room just the right shape for a swallow to sit in
while she brooded her eggs? How large would it have to be inside, to
hold four or five young swallows grown big enough for their first
flight? How thick would the walls have to be to make it strong enough?
What sort of curve would be best for its support against a perfectly
straight wall? How much space would have to be allowed for lining the
room, to make it warm and comfortable? How can the clay be handled so
that the drying sun and wind will not crack the walls? What is the test
for telling whether the clay is sticky enough to hold together? How much
of the nest must be stuck to the cliff so that the weight of it will not
make it fall?

If the architect can answer all those questions, ask him one more: ask
him if he could make such a nest with the same materials the birds used,
and with no more tools?

Well, Eve and Petro could and did. It was big enough and strong enough
and shaped just right; and when it was nearly done and nearly ready for
the soft warm lining, That Boy climbed the ladder and knocked it down