New Caledonian corvids, already considered the world’s smartest birds, now have even more to crow about. A new study found that their understanding of water displacement matches that of 7-10 year olds.

Earlier this year, a study found that Aesop’s Fable of the crow that uses pebbles to raise the water level of a pitcher to allow it to drink actually undersells their capacities.

Dr Corina Logan, of UC Santa Barbara has now taken this a step further. With work published in PloS One, "We showed that crows can discriminate between different volumes of water and that they can pass a modified test that so far only 7- to 10-year-old children have been able to complete successfully. We provide the strongest evidence so far that the birds attend to cause-and-effect relationships by choosing options that displace more water,” says Logan.

Intelligence is a well known feature of corvids, but the New Caledonian crows have advanced further than any others, leading the University of Auckland to establish a set of aviaries on the island to study their capacities. Although Logan worked from one of these aviaries she wanted to avoid using crows that might have learned too much from previous testing, saying, "We caught the crows in the wild and brought them into the aviaries, where they habituated in about five days."

Logan used one wide and one narrow beaker of water with the same size lids. "The question is, can they distinguish between water volumes?" Logan said. "Do they understand that dropping a stone into a narrow tube will raise the water level more?" In a previous study the birds were not able to work out that it was more efficient to drop stones in the narrow tube in order to raise the water level.

However, Logan saw a flaw – the crows were given 12 stones, enough to raise the water level sufficiently in either tube. The only reason to prefer the narrow tube was that it was quicker. "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,” Logan says.

Under such circumstances the birds realized they needed to use their resources wisely, and most did so, putting the pebbles in the narrow tube.

In another test Logan used the U-tube apparatus where dropping stones on one side of the tube raised the water level in the other, while in another, superficially similar, piece of equipment the same technique failed.

Children have been shown not to be able to fathom such hidden mechanisms until the age of 7-10. The crows failed previous tests, but when Logan moved the two sets of equipment further apart, one of her crows, Kitty, worked them out. At 6 months old, Kitty is well short of fully grown, but demonstrated her intelligence frequently, entering the testing area when Logan pointed to her or called her name, but not when others were invited. Not all the adult crows did likewise.

Logan admits researchers don't know the methods crows use to work out these tasks, nor the evolutionary reason why New Caledonian crows are smarter than their cousins elsewhere. "What we do know is that one crow behaved like the older children, which allows us to explore how they solve this task in future experiments."

Logan's research is part of her work to compare crows to grackles, birds that are thought to be almost as smart as crows, but with far smaller brains.

Previous experiments on New Caledonian crow intelligence by Sarah Jelbert, second author on this paper.