Aesop's fables are full of talking frogs and mice who wear clothes, but it turns out at least one of the classic tales is scientifically accurate.

Researchers presented four crows with a challenge from Aesop's fable "The Crow and the Pitcher": a container of water not quite full enough for the birds to reach with their beaks. Just like Aesop's crow, all four birds figured out how to raise the water level by dropping stones into the glass. The crows also selectively chose large pebbles over small ones, and quickly realized that dropping rocks into a container of sawdust didn't have the same effect.

"The results of these experiments provide the first empirical evidence that a species of corvid is capable of the remarkable problem-solving ability described more than two thousand years ago by Aesop," wrote the researchers in the paper published Thursday in Current Biology. "What was once thought to be a fictional account of the solution by a bird appears to have been based on a cognitive reality."

The researchers took four adult rooks, a type of intelligent crow, and tempted them with a tasty worm floating on top of a glass of water, just out of reach. Then they placed a pile of small rocks next to the crows. After they assessed the height of the water from the top and sides of the glass, the crows dropped stones into the glass until the water level rose enough for them to grab their prize.

Once they'd caught the worm, the birds didn't keep putting stones in the glass, and they didn't try to grab the worm until they'd dropped in a certain number of stones. "This number was strongly correlated to the number of stones needed to raise the water level to the correct height," the researchers wrote, "suggesting that, having assessed the starting level of the water, rooks translated this into an estimate of the number of stones needed."

Before this experiment, the birds had never been exposed to a glass with water in it, and they'd never used stones as tools. According to the researchers, the only other animal known to perform this kind of task is the orangutan, which has been recorded spitting into a tube to bring a peanut into reach.

"Corvids are remarkably intelligent, and in many ways rival the great apes in their physical intelligence and ability to solve problems," said biologist Christopher Bird of the University of Cambridge in a press release. "This is remarkable considering their brain is so different to the great apes'."

The antics of the four birds — Cook, Fry, Connelly and Monroe — can be seen in the videos below. Cook and Fry snagged the floating worm after just one try, while Connelly and Monroe succeeded after two attempts. Unfortunately, Fry had a bad reaction to one of the worms and gave up in the middle of the experiment.

Connelly captures the worm:

Monroe picks out large stones instead of small stones:

Cook has realized that sawdust doesn't work as well as water:

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Image 1: Flickr/jpmckenna. Image 2: Illustration by Milo Winter from The Aesop for Children/Project Gutenberg etext 19994.

Videos: Christopher Bird and Nathan Emery/Current Biology.