Birds of the crow family are widely considered to be intelligent animals and the New Caledonian crow has been shown to be as proficient in its use of tools as most of the great apes. In a recent article in Proceedings of the National Academy of Sciences of the USA, the aptly named Christopher Bird, together with Nathan Emery, has revealed that in the laboratory, rooks can show a level of tool use that is amongst the highest in the animal kingdom. Even more intriguingly, rooks in the wild have never been reported to use tools.
Bird and Emery presented hand-reared rooks with a series of experimental challenges that were carefully designed to explore the exact nature of the birds' abilities. First, the rooks were presented with a vertical tube in which a tasty but inaccessible worm could be retrieved by tipping the platform on which it was placed, by dropping stones down the tube.
The rooks were able to select stones of the right size and shape to fit into the tube, and would also go and get appropriate stones if none were provided near the apparatus. These behaviours indicate that the birds were able to select tools that were functionally relevant to the task. If the rooks were not given access to stones, but instead to a novel stick, all the birds immediately picked up the stick and solved the problem using this new tool. When provided with a combination of a functional stick and a non-functional stone (or vice versa), the birds chose the tool that would enable them to get the worm, reinforcing the interpretation that the functional aspect of the tool was fundamental for their choice.
Amazingly, the rooks were also able to show `metatool use', in which a tool was used to get access to a tool that would, in turn, gain access to the worm. This reveals that the rooks could recognise that a tool can be used on a non-food item, and that they were capable of hierarchically organizing their behaviour, resisting the temptation to try and get the food immediately. This is something that monkeys find extremely difficult. Success rates in this test were very high – nearly 97% and all birds solved it on the very first trial.
Finally, the authors tested the ability of rooks to make their own tools,either by removing side-branches from a stick, so that it would go into the tube, or by making a hook that would enable them to get the worm directly (the birds had already shown they could use an existing hook to this end). In both cases the birds showed spontaneous tool creation, manufacturing a hook or changing the shape of the stick. Their behaviour in this respect rivals that of the New Caledonian crow, and is superior to that of virtually every other animal that has been tested, with the exception of the great apes.
The particularly intriguing part of this study is that while rooks may be clever, there is no previous evidence of them using tools, either in the laboratory or in the wild. Similar findings have been reported in capuchin monkeys, which very rarely use tools in the wild but will do so readily in the laboratory. So what do rooks normally use their brainpower for? Why did the mental structures that underlie this behaviour evolve?
Either rooks do indeed show forms of tool use in the wild, and these have simply not been recognised, or their highly developed social foraging is the selective basis for their intelligence, and this can be applied to a variety of problems, including solving cunning experiments. By showing that natural tool use and advanced intelligence can apparently be separated, this study raises important questions about where, how and why animal intelligence evolved.