In New Caledonia, an island off the eastern coast of Australia, a crow is hunting for beetle grubs. The larvae are hidden within a decaying tree trunk, which might seem like an impregnable fortress. But the New Caledonian crow is smarter than the average bird. It uses a stick to probe the tunnels where the grubs are sheltered. The grubs bite at intruders with powerful jaws but here, that defensive reflex seals their fate; when they latch onto the stick, the crow pulls them out.
This technique is not easy. Birds need a lot of practice to pull it off and even veterans can spend a lot of time fishing out a single grub. The insects are fat, juicy and nutritious but do they really warrant the energy spent on extracting them? The answer is a resounding yes, according to Christian Rutz from the University of Oxford. By analysing feather and blood samples from individual crows, he found that grubs are so nutritious that just a few can satisfy a crow for a day.
Aesop’s fable “The Crow and the Pitcher” has been confirmed in a wonderful experiment. In the classic tale, a thirsty crow uses stones to raise the level of water in a pitcher until it rises within reach of its beak. This is no mere fiction – rooks, close relatives of crows, have the brains to actually do this.
The aptly named Chris Bird, along with Nathan Emery, gave four captive rooks (Cook, Fry, Connelly and Monroe) a chance to reach a small worm floating in a cylinder of water, with nothing but a small pile of stones sitting on the side. All of them solved the task, and Cook and Fry succeeded on their first attempt. They were savvy about the stones too, using exactly the right number to bring the water within reach and preferring larger stones over smaller ones.
The accomplishments of Bird’s rooks are even more impressive when you consider that rooks are not natural tool-users. Many of the corvids – crows, ravens and the like – are avid tool-users and the skills of the New Caledonian crow are rapidly becoming the stuff of popular science legend. But rooks are different – even though they too excel in laboratory tests (as Bird and Emery have previously shown), they hardly ever use tools in the wild. Rather than any special tool-using adaptations, their skills must stem from the sort of general intelligence that great apes are thought to possess.
More so than other members of the family, rooks are extremely opportunistic feeders, relying on a varied menu of seeds, insects, dead meat and rubbish. With such catholic tastes, food is never far from their beaks and they may have little need for specialised tricks involving tools. The same might be true for capuchin monkeys, which happily brandish tools in a lab but only ever use them in the wild when food is scarce. As Aesop’s fable moralised, “Necessity is the mother of invention.”