a good many
very different things are called teeth--those on a rake, for example, or
a comb, or a cog-wheel. A Duck's teeth are horny like the skin that
covers its whole beak, and act like strainers. When a Duck dabbles in
the water, as you have all seen tame ones do, the water that gets into
its mouth runs out at the sides between the teeth, but whatever food
there is in the mouthful of water gets caught in the teeth, and can then
be swallowed."

"Please tell us," continued Rap, "how many different kinds of Ducks
there are in our country?"

"About forty," answered the Doctor; "but I shall not trouble you to
learn more than a few of the common ones. They all belong to one family,
which also contains the Geese and Swans. They are divided into three
groups--Fishing Ducks, River or Fresh-water Ducks, and Sea Ducks.

"The Fishing Ducks are great swimmers and divers, living chiefly on the
fish they catch by chasing them under water. Their beaks are narrow,
hooked, and sharply toothed, which makes it easy for them to hold their
slippery prey. But this oily food makes their flesh so rank that none of
them is fit food for House People. They are all called Mergansers, and
we have in this country four different species.

"The River Ducks are those that we see mostly in the spring and fall
migrations; they have the handsomest plumage and the most delicate
flesh. They feed along shallow rivers, ponds, and lakes, after the
manner of barnyard Ducks--for the Mallard is one of them, and tame Ducks
are domesticated Mallards, as I told you. In feeding, they bob head
downward in the water with their tails straight up in the air, to find
the roots, seeds, insects, small shell-fish, and other things they like
to eat. They build very good nests, usually on the ground, and warmly
lined with their own down, which the parent plucks from her breast to
cover the eggs. The color of the eggs is always greenish, gray, drab, or
buff, never with any spots. Most River Ducks nest in the far North, but
there are some exceptions. The Wood Duck that Rap saw by the lake is one
of these exceptions, and has the most beautiful plumage of all our
Ducks. It does not build its nest on the ground, like most others of its
family, but in a tree hole, like an Owl or a Woodpecker."

"How can the little Ducks get down to the ground--do their wings grow
strong very soon?" asked Nat.

"You have seen that most birds come from the egg quite naked, and stay
in the ne