Clever crows rely on a unique bird's eye view, researchers discover
Scientists at the University of Birmingham studying New Caledonian crows have discovered why these birds, which are famed for their intelligence, are able to use tools with such accuracy. The answer lies in their vision, according to research published today (9 October 2012) in the journal Nature Communications.
These crows have a very wide binocular field of vision and a straight bill which together enable them to position their tools with the high accuracy that is necessary for finding food hidden in otherwise inaccessible places,
New Caledonian crows, a species native to an island in the South West Pacific Ocean, are well known for their use of tools for extractive foraging. With a remarkable degree of dexterity they build complex tools out of twigs and leaves and insert them into narrow holes in deadwood to extract beetle grubs.
It has always been thought that the New Caledonian crow’s ability to craft and manipulate tools for foraging could be put down solely to their superior intelligence. However, this study shows that two other attributes are essential for such complex tool making and tool use.
The researchers discovered that these birds have an extremely wide field of binocular vision, as well as a straight bill; a combination of features which the researchers did not find in other crow species. The New Caledonian Crows hold the tool in their bill tip and prop one end of it against their cheek. This ensures that the tool is held firmly but projecting slightly to one side. Their straight bill means that the tool is held up high close to their line of sight and their wide binocular visual field crucially allows them to see along the bill shaft and position it accurately when probing.
Graham Martin, Professor of Avian Sensory Science at the University of Birmingham, said: ‘The New Caledonian crows have been recognised to be highly intelligent because of their tool making skills, but we thought that these birds must have other features which help them to finely control the tool and to apply it so precisely.
‘We have learnt, from studying these crows, that they have other characteristics - a straight bill and wide field of vision that enable them to be so dextrous and accurate when manufacturing their tools and using them so successfully in foraging. The unusually straight bill raises the tool tip into their visual field’s binocular area and this gives them accurate control of bill position. These are features which we have shown are lacking in other crow species. Their absence is probably the key reason why other birds of the crow family, such as Jackdaws, Rooks and Ravens do not use tools. It may not be a lack of intelligence that has inhibited the evolution of tool use in these other crows. It looks as though a firm grip and a wide binocular field are necessary to allow a bird to see straight down the line of the tool and place it with precision.
‘We take it for granted that we can use tools accurately, but we have hands and a wide binocular field. The result is that we can always place tools in our line of sight and so position them accurately. Without hands birds need some additional features for successful tool positioning. Among birds these features seem to have evolved uniquely in New Caledonian crows.’
Dr Jackie Chappell, Lecturer in Animal Behaviour, University of Birmingham, said: ‘Having hands that we can move independently of our eyes gives humans a big advantage when using tools. Tool-using birds face a much more difficult problem because they hold the tool in their bill: imagine having your eyes located on your hands when using a screwdriver. Our research has shown that - uniquely among crows - New Caledonian crows' bill shape and wide binocular field helps them to overcome these problems and to hold the tool steady while being able to see what they are doing with it. As far as we know, this is the first evidence for tool-use-related morphological features outside of the human lineage.’
Notes to Editors
The research is published in the journal Nature Communications, ‘Extreme binocular vision and a straight bill facilitate tool use in New Caledonian crows.’ DOI 10.1038/ncomms2111
Jolyon Troscianko , Auguste M.P. von Bayern , Jackie Chappell, Christian Rutz & Graham R. Martin.
For further information
Kate Chapple, Press Officer, University of Birmingham, tel 0121 414 2772 or 07789 921164.