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Planet Earth

Fishing for fat: why learning to use tools is worth it for the New Caledonian crow

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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.

The New Caledonian crow is one of the most accomplished tool users of all animals. They produce at least four different types from sticks and grass stems. Their tools have complicated shapes and are manufactured through a process that involves several steps. They can pick the best tools for the job, adjust their size and shape to suit their needs, combine them in sequence to complete a task, and create “meta-tools” by using one tool to get at another. They can make tools out of unfamiliar materials, like coat hangers.

We know this mostly through experiments with captive crows. In the wild, the crows are secretive and easily startled so the rewards they gain from their mighty intellects are unclear. Rutz has studied them with video cameras that were triggered by movement, and his movies showed how difficult it is for a crow to extract a grub, even if it’s experienced.

Filming the birds non-stop would be impossible, but fortunately, Rutz didn’t have to. He could work out what the birds were eating by collecting blood and feather samples, and measuring the ratio of carbon and nitrogen isotopes (slightly different versions of the same atoms). These values reflect the chemical make-up of the crows’ food, each of which has a different isotope ‘signature’. As the saying goes, you are what you eat; the crows’ bodies betrayed their dietary choices.

Crow_food.jpg

The crows eat seven different types of food – grubs, nuts, carrion, lizards, snails, fruit and small invertebrates. In terms of isotopes, beetle grubs stand out. They eat wood and they digest it with special bacteria in their guts, which dramatically sway their ratio of nitrogen isotopes.

Rutz found that the grubs provide the crows with about as much protein as all the other sources, but they contribute almost half of the fat in the birds’ diet. On average, the crows eat just under two grubs a day, and Rutz calculated that they would only need 3 to meet their entire energy needs.

This shows that an enterprising crow gets tremendous benefits from investing the time and effort it takes to use tools properly. Indeed, young crows can spend years fiddling with their tools to no avail. It’s an unrewarding period, but it eventually pays off.

The presence of such an untapped source of energy may have spurred the evolution of the New Caledonian crow’s tool-making prowess. But Rutz thinks that early humans may have inadvertently nudged this process along. On New Caledonia, beetle grubs burrow into the logs of decaying candlenut trees, but these trees were only introduced to the island by prehistoric humans. The trees also provide nuts – the second most important source of fats and proteins for the crow. Rutz suggests that this sudden influx of nutrients might have been a key event in the evolution of one of the world’s smartest birds.

Reference: Science http://dx.doi.org/10.1126/science.1192053

More on clever corvids (crows and their relatives)

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