For many years, researchers thought that the division of labor in the brain, known as cerebral lateralization, was unique to humans. But recent research has shown that such lateralization is actually pervasive in vertebrates. A leading theory suggests that the attribute leads to faster, more accurate problem-solving. The theory holds true for minnows--the ones whose brains are lateralized are better at catching shrimp while simultaneously keeping an eye out for predators--but many other species haven't been tested.

Among birds, parrots and crows are renowned for their cleverness. So behavioral ecologist Culum Brown and biologist Maria Magat of Macquarie University in Sydney, Australia, tested 40 parrots from eight different Australian species. Just as right-handedness indicates left-brain dominance in most humans, brain laterality was determined in birds by observing which eye each bird used to fixate on a piece of food and which foot grabbed it. Each bird received a laterality score ranging from 0 (no preference) to 5 (strongly lateralized).

The parrots were then given two tests. One involved picking out seeds from a background of similar-looking pebbles; their performance was evaluated by dividing the number of seeds consumed by the number of pecks. The more challenging task required birds to obtain food hanging below their perch on a 50-centimeter-long string. Hauling up the prize is a problem requiring a lot of beak, foot, and eye coordination.

Single-minded...Culum Brown, whose observations of parrots have shown a link between intelligence and the preference for one hand over the other. Photo: Jon Reid

Today in the Proceedings of the Royal Society B, the researchers report that birds with stronger brain asymmetries tended to be more successful. Cockatoos tended to be the brightest and budgerigars the dimmest, but within species there was variability according to the degree of laterality. In the string test, for example, five strongly lateralized birds (one right-footed and the rest left-footed) from four species succeeded on the first try. Birds with no lateralization performed the worst--in the pebble test they scored 55% compared to 95% in the strongly lateralized individuals. "These individuals have problems with coordination," says Brown. "They try a mixture of approaches, and sometimes they manage to muddle through it."

The authors say the experiments show that cerebral lateralization promotes fast and accurate thinking and coordinated movements--and hence would increase the birds' fitness, or ability to survive and reproduce.

Cognitive neuroscientist Giorgio Vallortigara of the University of Trento in Italy, who has studied performance of chicks on the seed-pecking test, says, "The idea of a link between lateralization strength and cognitive abilities has been around ... for many years, but little comparative and experimental work has been done with animals." This study, he says, provides "fascinating confirmation of the link between higher cognition and brain asymmetry."