Aesop's ancient fable The Crow and the Pitcher tells the story of a thirsty bird who cleverly drops stones into a pitcher of water, raising the liquid's level to quench his thirst. Now, scientists have shown that rooks (Corvus frugilegus), birds from the crow family, can perform a very similar task. The finding suggests that facts may underlie the legend and indicates that corvids, like great apes, have a general understanding of physical rules.
"I had always wanted to see if there was a way to test what the crow did in Aesop's fable," explains Nathan Emery, a comparative psychologist at Queen Mary, University of London. But he and his graduate student, Christopher Bird of the University of Cambridge, realized they couldn't "ethically deprive a bird of water." So they devised an experiment that would approximate the challenge facing Aesop's crow. In previous experiments, the two had shown that the rooks, which are not known for using tools in the wild, would nevertheless pick up stones and drop them into a tube in order to make a treat roll out. In the new experiments, Emery and Bird gave each of four rooks (two mated pairs) a clear plastic tube; the tube contained the larvae of a wax moth--the birds' favorite food--floating near the bottom, just beyond the reach of the rooks' beaks.
Videos of the tests show the birds moving and cocking their heads to look at the worm from the side and top of the tube, "sizing up the problem," as Emery puts it. The researchers then placed a small pile of stones next to the tube; in some of the experiments, these varied in size, so the birds had a choice of using either large or small stones. The amount of water in the tube also varied, requiring that the birds drop between one and seven rocks in order to get the prized worm.
Two of the rooks solved the problem on their first try; the two others only dropped in a few stones on their first two tests, then gave up, but later persisted and got the worm. When given a choice of stones to use, all the rooks quickly determined that large stones were the best, and they efficiently added just enough stones to raise the water to the right height, the researchers report today in Current Biology. "It's as if they were estimating the number of stones they needed right from the start," says Emery, adding that the birds never "just went straight in, but always seemed to think the problem through."
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"This is a truly remarkable study," says primatologist Frans de Waal of Emory University in Atlanta. He says that the rooks' solution reminds him of experiments in the 1920s, which showed that chimpanzees could stack wooden crates to reach a banana. Orangutans also do something similar: spitting water into a tube to snag a peanut. It's as if "a light bulb goes on" in the birds' brains, de Waal says, as they "work out the solution in their heads."
Emery and Bird argue that their experiments are a further demonstration of convergent cognitive evolution between the primate and corvid lineages, with both groups having a generalized understanding of the physical world. The rooks' behavior may not qualify as "insightful"--a higher cognitive function usually attributed only to humans--but it comes close, says Bird, because they are "clearly combining some sort of understanding of the task with an understanding of the tool and are able to solve the task so quickly." At the very least, adds Emery, the experiments demonstrate the rooks' talent for "innovation, because they are adapting their previous experience with stones and tubes [in other experiments] to a new problem." The rooks, it seems, have thus even substantiated the closing moral to Aesop's crow fable: "Necessity is the mother of invention."