"Volpla", a 1956 novelette by Wyman Guin, was also the answer to this old question. The text is available at Project Gutenberg; the X Minus One radio adaptation is available at the Internet Archive. Does any of these covers ring a bell?
A scientist encloses a community of mice inside a machine that makes time go much faster inside the machine.
He calls the machine a "metabolic accelerator":
Again in the laboratory, I entered the metabolic accelerator and withdrew the intravenous needles from my first volplas. I carried their limp little forms out to a mattress in the lab, two girls and a boy. The accelerator had forced them almost to adulthood in less than a month. It would be several hours before they would begin to move, to learn to feed and play, perhaps to learn to fly.
Thus he can breed successive generations of mice incredibly quickly.
Rats, not mice:
The fact was that I had something more in the lab than I had bargained for. I had aimed only at a gliding mammal a little more efficient than the Dusky Glider of Australia, a marsupial. Even in the basically mutating colony, there had been a decidedly simian appearance in recent years, a long shift from the garbage-dump rats I had started with. But my first volplas were shockingly humanoid.
He breeds the population to have wings
When the male spread his arms, the span was forty-eight inches. I held his arms out and tried to tease the spars open. They were not new. The spars had been common to the basic colony for years and were the result of serial mutations effecting those greatly elongated fifth fingers that had first appeared in Nijinsky. No longer jointed like a finger, the spar turned backward sharply and ran alongside the wrist almost to the elbow. Powerful wrist muscles could snap it outward and forward. Suddenly, as I teased the male volpla, this happened.
The spars added nine inches on each side to his span. As they swept out and forward, the lateral skin that had, till now, hung in resting folds was tightened in a golden plane that stretched from the tip of the spar to his wrist and continued four inches wide down his legs to where it anchored at the little toe.
This was by far the most impressive plane that had appeared till now. It was a true gliding plane, perhaps even a soaring one. I felt a thrill run along my back.
and to be intelligent.
Yes. The scientist is planning a practical joke on the world:
I sipped at my martini and lounged in a terrace chair watching the golden evening slant across the beautiful hills of our ranch. I dreamed. I would invent a euphonious set of words to match the Basic English vocabulary and teach it to them as their language. They would have their own crafts and live in small tree houses.
I would teach them legends: that they had come from the stars, that they had subsequently watched the first red men and then the first white men enter these hills.
When they were able to take care of themselves, I would turn them loose. There would be volpla colonies all up and down the Coast before anyone suspected. One day, somebody would see a volpla. The newspapers would laugh.
Then someone authoritative would find a colony and observe them. He would conclude: "I am convinced that they have a language and speak it intelligently."
He is able to communicate with them but discovers they are curious about their heritage. He invents a mythology that their race comes from the planet Venus.
He looked up at the night sky and, in the firelight, I saw his wonder. "You say we came from there?"
"The old ones of your kind told me so. Didn't they tell you?"
"I can't remember any old ones. You tell me."
"The old ones told me you came long before the red men in a ship from the stars." Standing there in the dark, I had to grin, visioning the Sunday supplements that would be written in about a year, maybe even less.
He looked into the sky for a long time. "Those little lights are the stars?"
"That's right."
"Which one?"
I glanced about and presently pointed over a tree. "From Venus." Then I realized I had blundered by passing him an English name. "In your language, Pohtah."
He looked at the planet a long time and murmured, "Venus. Pohtah."
They somehow obtain a space-craft and take off
"Just as I pushed the button and the hatch was closing, a flock of owls circled the ship. They started flying through the hatch and somehow they jammed it open."
Em said to my wife, "There must have been a hundred of them. They kept coming and coming and flying into that hatch. Then they began dumping out all the recording instruments. The men tried to run a motor-driven ladder up to the ship and those owls hit the driver on the head and knocked him out with some kind of instrument."
Guy turned his grief-stricken face to me. "Then the hatch closed and we don't dare go near the ship. It was supposed to fire in five minutes, but it hasn't. Those damned owls could have—"
There was a glare in the east. We all turned and saw a brief steak of gilt pencil its way up the black velvet beyond the mountains.
"That's it!" Guy shouted. "That's the ship!" Then he moaned. "A total loss."
I grabbed him by the shoulders. "You mean it won't make it to Venus?"
He jerked away in misery. "Sure, it will make it. The automatic controls can't be tampered with. But the rocket is on its way without any recording instruments or TV aboard. Just a load of owls."
My son laughed. "Owls! My dad can tell you a thing or two."
in search of what they believe to be their 'home' planet.
For a moment there was only a gritty buzz from the receiver. Then the tape started playing a soft, high voice. "This is Rocket Harold saying everything is well. This is Rocket Harold saying good-by to men." There was a pause and then, in clear volpla language, another voice spoke. "Man who made us, we forgive you. We know we did not come from the stars, but we go there. I, chief, give you welcome to visit. Good-by."
