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.