We know that crows are capable of some pretty complex problem solving. Earlier this year, we saw a crow figure out how to complete an eight-step puzzle to retrieve food, demonstrating the supreme reasoning capability of corvids.

That crow, however, had already been shown each of the steps. Not in order -- the crow put them together on its own, which is still a highly impressive feat -- but it knew which items triggered which effects. Now, it seems, that crows are capable of figuring out at least one of the steps: how to get water from a tall glass.

To you or me, the solution seems simple: drop rocks into the glass to raise the water level. One would not usually expect a bird to be able to figure this out, but -- as has been demonstrated in the past by Sarah Jelbert at the University of Auckland-- crows can. Moreover, they can differentiate between a floating object and a sinking one, and can tell the difference between sand and water.

New research, conducted by a team of researchers at the University of California, Santa Barbara led by Corina Logan, has shown that they can distinguish between water volumes, too.

Using New Caledonian crows caught in the wild, the team presented them with two volumes of water, one in a wide beaker of water, the other in a narrow one. Both beakers were the same height. However, unlike Jelbert's research -- where the crows were given enough stones to succeed in raising the water level to a desired height no matter which vessel they chose -- Logan's team only gave them enough for one.

"When we gave them only four objects, they could succeed only in one tube -- the narrower one, because the water level would never get high enough in the wider tube; they were dropping all or most of the objects into the functional tube and getting the food reward," Logan said. "It wasn't just that they preferred this tube, they appeared to know it was more functional."

The other test, called the U-tube test, involved sets of tubes. One set is connected by a hidden mechanism; when stones are dropped in one tube, water rises in the other. The other set of tubes is unconnected, so dropping stones in one produces no result in the other. Each set of tubes is colour-coded so that the test subject can differentiate between them.

While children aged 7 to 10 can figure out the simple rules, children aged 4 to 6 cannot; and, when Jelbert tried it with her crows, they failed. Logan decided to re-attempt the test, moving the beakers farther apart -- and one of the crows, a six-month-old nicknamed Kitty, figured it out.

"We don't know how she passed it or what she understands about the task," Logan said, "so we don't know if the same cognitive processes or decisions are happening as with the children, but we now have evidence that they can. It's possible for the birds to pass it."

The full paper, "Modifications to the Aesop's Fable Paradigm Change New Caledonian Crow Performances", can be read online in the journal PLOS One.