is kept up endlessly."

"I can see why they go south," said Rap, after thinking a few moments,
"but why do they come back again? Why don't they stay and build their
nests down there?"

"That is a difficult question to answer," said the Doctor, "and one that
we House People try to explain in different ways. I think that the love
of the place where they were born is strong enough in birds to bring
them back every season to build their nests. So you see that Citizen
Bird is a patriot; for, though he may be in the midst of plenty in a
tropical forest, when the time comes he travels hundreds of miles to his
native land to make the young, that will fly from his nest, citizens
like himself."

"But the birds that can eat seeds and other things do not travel so far,
do they?" asked Rap.

"No, the birds who rove about the United States throughout the year are
either Weed Warriors, or Seed Sowers, or those Tree Trappers who creep
about tree-trunks picking the eggs and grubs of insects from the bark.
Or else those great Cannibal Birds, the Wise Watchers, who eat the flesh
of their smaller brothers, as well as of rats, mice, and all such
vermin--the Hawks and Owls; or else they are Gulls, Terns, Fishing
Ducks, and a great many other kinds of sea birds who feed on fish and
pick up the scraps floating on the surface of sea, lake, and river."

"Do the Barn Swallows that are making nests in the hayloft go as far
south as Kingbirds?" asked Nat.