In order to test the birds' abilities, the researchers examined manufacture techniques in forests across New Caledonia. Situated in the Pacific Ocean east of Australia, the French islands of New Caledonia contain a unique and isolated group of species.
In addition to twigs, the crows manufacture tools from long and barbed leaves of the pandanus (or screw pine) tree.
Crows snip into the leaf edges and then tear out neat strips of vegetation with which they can probe insect-harboring crevices. These tools have been observed to come in three types: narrow strips, wide strips and multi-stepped stripswhich are wide at one end and, via a manufacturing process that involves stepwise snips and tears, become narrow at the opposite end.
Instead of attempting to locate crows using pandanus leaf toolswhich would have been a logistical challengeHunt and Gray scoured the forests for so-called leaf "counterparts." A counterpart is the exact mirror-image outline, or carbon copy, of the tool's shape left behind in pandanus leaves still attached to the tree.
By examining more than 5,500 tool imprints in 21 sites throughout New Caledonia, the pair were able to assess patterns in the tool design and location and abundance of each tool type. The results suggested that both narrow and stepped tools are more advanced versions of the wide tool type.
No Similar Example
Tool invention is very rare, said Hunt. Therefore it's quite unlikely that each type of leaf tool has been the result of a unique discovery. In addition, the geographical distribution of each tool type on the island suggests a unique origin, rather than multiple independent inventions.
The crow has "developed the capacity to evolve its tools," said Hunt, an important step in the direction of a complex material culture, like that exhibited by people.
This is an extremely interesting and well conducted study, commented Alex Kacelnik behavioral ecologist at the University of Oxford in England. There is no similar [non-human] example of cumulative transmission of a skill such as the making of pandanus [tree] leaf tools by New Caledonian crows, he said.
Kacelnik is one of the co-authors behind last year's report detailing the striking tool-making capabilities of the captive crow called Betty.
Unfortunately, We still don't know much about the process of cultural transmission and hence of what is special in these animals, to make them the master craftbirds that they obviously are, he said.
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