s called the
Rock Wren--"

"Oh! I know--because he lives in the Rocky Mountains," said Dodo,
clapping her hands at this discovery.

"Yes, that is partly the reason," resumed the Doctor, after this
interruption, "but those mountains are very many, and varied in
appearance, like most others: covered in most places with pine trees,
but including in their recesses grassy meadows and silvery lakes. Some
parts of those mountains are the home of the Rock Wren, but the little
fellow is quite as well satisfied anywhere else in the western parts of
the United States, if he can find heaps of stones to play hide-and-seek
in with his mate, or great smooth boulders to skip up to the top of and
sing. So you see the mountains and the Wrens are both named for the
rocks.

"Do these Wrens look like our kind and act that way?" asked Nat. "Ours
always make me think of mice."

"All kinds of Wrens are much alike," answered the Doctor. "They are
small brownish birds with cocked up tails, not at all shy about showing
themselves off, when they choose, but they must have some hiding-place
to duck into the moment anything frightens them, and some odd,
out-of-the-way nook or cranny for their big rubbishy nests. Some prefer
to hide in marshes among the thickest reeds, some live in dry brush
heaps, and some, like the Rock Wren, choose piles of stones. Their wings
are not very strong, and they seldom venture far from their favorite
retreats, except when they are migrating.

"When your cousin Olive and I were in Colorado we climbed a mountain one
day above the timber-line"--

"Do _all_ the trees out there grow in straight lines?" asked Dodo
anxiously.

[Illustration: Rock Wren.]

"No, my dear little girl, trees don't grow in straight lines anywhere,"
said the Doctor, laughing--"except when they are planted so. The
'timber-line' of a mountain is the edge of the woods, above which no
trees grow, and we see nothing but bare rocks, and the few low plants
that cling to the cracks among them. Well, we had hardly rested long
enough to get our breath after such a climb, when we heard a rich
ringing song, something like a House Wren's, but louder and stronger,
and very quick, as if the bird were in a great hurry to get through. But
he wasn't, for he kept saying the same thing over and over again.
Presently we spied him, on the tiptop of a pile of stones, standing
quite still, with his head thrown back and his bill pointing straight
up. He looked g