Subverting some popular thoughts on tool use and physical intelligence, a group of captive rooks has shown a remarkable capacity to modify and use various tools. Rooks (Corvus frugilegus) are not known to use tools in the wild, although they are closely related to New Caledonian crows (Corvus moneduloides), which are habitual tool users.

Habitual tool use was once thought to be exclusive to humans, then later identified in other primates and some bird species: chimpanzees, orangutans, New Caledonian crows and woodpecker finches. The use of tools is thought to have evolved in crows as a result of cognitive specialization, ecological pressure or morphology. But the new data on rooks suggests that tool use in this group of birds may have developed from an advanced physical intelligence rather than as an adaptive specialization.

Credit: Andrew Howe

Christopher Bird (University of Cambridge, UK) and Nathan Emery (Queen Mary University of London, UK) reported the rooks' spontaneous tool use (Proc. Natl. Acad. Sci. USA published online 28 May 2009; doi:10.1073/pnas.0901008106). They designed a testing apparatus in which a food reward was placed on an inaccessible platform inside a clear box. The rooks could retrieve the reward only if they knocked the platform down by dropping an appropriately sized and weighted object through a clear tube mounted on top of the box.

Rooks were first presented with this apparatus along with a stone positioned so that it could be nudged into the tube. After successfully completing five 'nudging' trials, the birds were given five more trials in which they had to retrieve a stone from near the base of the tube before dropping into the tube. Each bird was removed from the aviary after succeeding at this 'transfer' task five times to allow another bird to attempt the task. One bird needed no 'nudging' trials; after watching another rook complete the transfer task four times, she spontaneously picked up the stone and dropped it down the tube.

The first four rooks to complete five transfer trials were subjected to further testing, which showed that they could select tools (stones, sticks or wire hooks) of the appropriate size and shape and orient them so that they could fit in the tube. Furthermore, they were able to use one tool in order to reach a second tool that they needed to retrieve the reward. They also modified tools when necessary to make them functional.