While the species of the crow family - including jackdaw, magpie, crow, rook and raven - are renowned in myth and legend for their intelligence, a scientific basis for this has been hard to establish. Until, that is, researchers discovered a crow species living on the South Pacific islands of New Caledonia that manufactured tools more complex even than those used by humans' closest relatives, the chimpanzee.
The original report from the 1990s described how the crows' tool making displayed three characteristics not seen before outside of humans: a high degree of tool standardisation, distinct tool designs that involve obvious imposition of form onto raw material, and the use of hooks. Sharing some traits with those of stone age humans' tools, this led to a surge of interest in the species.
Research over the past ten years has found a great deal of interesting behaviour in New Caledonian crows. They have been shown to make tools out of novel materials, use tools in novel ways to solve physical problems at which our close primate relatives fail, and to solve problems in a way that suggests abstract reasoning. Their impressive problem solving skills appear to be backed up by a relatively large brain, but more important than relative brain size is the neurological hardware inside it.
There is some evidence that the crows, like primates, have a relatively large forebrain where most decision making occurs. The crows and certain other birds have distinct cell clusters in the forebrain, consisting of a neuron surrounded by supporting glial cells. The density and distribution of these clusters in the forebrain varies between species, but the New Caledonian crow has a high density of them. If these clusters, as is believed to be the case in humans, play an important role in underpinning cognitive skills, they may help explain the crows' sophistication.
In a new volume on tool use as adaptation from the Royal Society, the New Caledonian crow figures prominently. One study questions whether the birds know that the hooked end of a hooked stick is the functional end.
A similar question has been asked in a previous study about the hooks they fashion from screw pine leaves. These have barbed edges, and the crows tear strips from the leaf edge to use as hooked tools to extract small prey hiding in trees. In that study the birds observed did not seem to care if the natural barbs on the edges of the leaf strips were absent or incorrectly orientated, even though a barbless or upside down tool would not do the job of extracting food. In contrast, the crows in the new study paid attention to the hooks, and mostly used the hooked end as the working end, findings which corroborate signs of wear seen on only the hooked ends of stick tools that have been made and used by the crows in the wild.
Another study found New Caledonian crows outperformed their more common relative the carrion crow in a task where they had to pull one of two sticks with curved ends to obtain food. Only one of the sticks was in an appropriate position when pulled to rake the food towards the crow. But the New Caledonian crows did worse than their cousins in two general learning tasks which didn’t require making decisions about relationships between objects. This suggests that New Caledonian crows' complex tool skills are underpinned by adaptations in their brains that specifically enhanced only their capacity to solve object-related problems.
Humans have co-evolved with tool use and have adapted to use them efficiently, from recognising how and why tools work to having the appropriate physical requirements - such as opposable thumbs - to make and manipulate them. It’s likely that New Caledonian crows are similar in having developed cognitive adaptations that support their tool-using lifestyle. These may take time to discover, but there is evidence that the crows exhibit both behavioural and morphological peculiarities that may have evolved with their tool use.
For example, parents feed their juveniles for an exceptionally long time – often up to ten months. This is rare in birds, and in those species where it occurs (certain seabirds and birds of prey, for example) it’s associated with the need for juveniles to learn complex foraging techniques.
Also unusual is the crows' exceptionally wide binocular vision, with eyes whose range overlaps to the front so they can see forward as well as sideways. Combined with an unusually straight bill, researchers have suggested this allows them to focus more clearly on the tool held in their bills, and manipulate it more exactly.
The New Caledonian crow is a valuable living, non-primate species from which to learn about the evolution of tool use in early humans. In contrast to our human ancestors, we can today study both the creature’s intelligence, and the complex tools it creates. As increasing evidence suggests the crow’s tool use is built upon various cognitive, behavioural and physical adaptations, this strengthens the idea that the crows have, like humans, been fine-tuned by evolution to become the remarkable technological specialists that we see today.