FROM ancient cave art in France to Van Gogh’s last painting, members of the crow family, or corvids, have inspired mythology and literature implying their intelligence to be superior to that of other animals. Indeed, corvids are cleverer than most birds, and are known to perform feats that can stump even primates and small children. Wild hooded crows in Israel, for instance, catch fish by dropping bread crumbs as bait. New Caledonian crows in New Zealand fashion hooks from twigs to capture food. While much existing research has focused on showing that these avian Einsteins use creative methods to solve complex tasks, surprisingly little is known about the limits to which their tiny brains can be pushed. But in a new study, published recently in PLOS ONE, Sarah Jelbert and her colleagues at the University of Auckland conducted a series of experiments on New Caledonian crows to explore just this. By recreating the setting from Aesop's Fable “The crow and the pitcher”, the researchers have shown that corvids are not just capable of grasping that solid objects displace water, but more crucially, they seem to understand the causal relationship between the properties of such objects and their effectiveness. Before the tests got under way, the researchers taught the birds to pick up stones with their beaks. The crows were then presented with two tubes, one half-filled with water and another with an equal proportion of sand. Both contained a cube of meat that was out of reach. The researchers observed that the crows dropped a large percentage of stones in the tube filled with water until their floating reward was at a beak's length. The sand-filled tube was mostly left alone.

Variations of these experiments showed that the crows consistently targeted the tube with higher water level as compared to the shallow one. They also chose to drop sinking objects like erasers over floating ones made from polystyrene as a ploy to raise the water level. Hollow objects, too, got short shrift when a more fruitful solid alternative of the same weight, colour and size was at hand.

How far can the birds fly with their brain power? To find out, when the researchers further cranked up the difficulty level of their tests, the New Caledonian crows faltered. On being offered two water-filled tubes of different dimensions, the birds preferred to drop solid objects in a tube with a wider base. Although they eventually snapped up their floating prize, it involved more grunt work.

A more complex test involved a tube with a rim so narrow that no stones could be dropped through it to access the food inside. This tube, however, was connected to another in a concealed manner such that dropping solids in it would raise the water level in the tube containing the piece of meat. A third unconnected tube with water, but no meat, was also thrown in. The crows could not decode the apparatus despite their trial and error approach.

But their failure to grasp the concept only proves that crows rely on causal reasoning to solve a riddle. The researchers conclude that the crow’s performance is comparable to that of five to seven year old children who, too, failed this test while passing others. As bird-brains go, crows’ are in a league of their own.