Planet Earth

Tool-Using Crows Overcome Their Lack of Pockets

InkfishBy Elizabeth PrestonJun 2, 2015 6:55 PM


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Let's say you're clever enough to build and use tools, but your species hasn't learned how to manufacture pants. So you can't store your hard-won tools in your pocket, or in a belt or box. What to do? One species of crow is showing scientists how it answers that question—and how it changes its strategy based on how likely its tools are to go missing. New Caledonian crows, native to islands in the southwest Pacific Ocean, are renowned tool makers and users. They prey on bugs that live inside branches and plants. To fish out their prey, the birds use sticks or leaves, which they may trim and tweak to fashion into hooks. Humans have sometimes noticed these birds trapping their tools under one foot while they munch on a bug, or stashing tools in convenient holes. It makes sense—after going to the trouble to MacGyver a fishing hook out of a plant, you wouldn't want to lose it either. And when the birds do lose their tools, they can look distinctly peeved. Watch the bird at the beginning of this video drop its stick and fly off in a huff: Biologists at the University of St. Andrews set out to see whether crows have a system for storing their tools. Graduate student Barbara Klump and her colleagues observed wild New Caledonian crows, capturing video of five birds using tools. They also tested nine other crows in the lab after trapping them outdoors. Since New Caledonian crows hunt for food under a wide range of conditions, the scientists thought the birds might have a range of strategies for keeping their tools safe. When birds were higher up, would they be more careful not to drop a stick? What if their prey were especially difficult to grapple with? In the lab, the researchers gave birds a branch lined with baited holes. A plant stem, which the birds could use as a tool, was conspicuously wedged into a nearby log. The researchers tested each bird with both a branch on the ground and one propped 1.3 meters high. They also gave birds both easy and difficult-to-handle prey (a plain cube of juicy meat versus one with a feather stuck through it). The birds videotaped foraging in the wild always kept their tools safe. Their preferred method was to trap a stick under one foot, but occasionally they put a stick into the hole they'd just pulled a bug from. In the lab, birds were also careful with their tools. And their stick-stashing methods depended on the circumstances. Birds were more likely to store their tools when they were on an elevated branch. On the ground, where dropping a tool wouldn't be a big deal, they were less cautious. On a high branch, they were more likely to stow tools in a hole, rather than underfoot. They also used holes more often when they were handling difficult prey, setting their tools aside to focus on getting the feather out of the meat. "I was surprised by the fact that the crows are in general so good at looking after their tools," Klump says. She was also impressed by the strength of the effect she found. Even though the "high" branch in the lab was less than a meter and a half off the ground, the birds on this branch were significantly more careful not to lose their sticks. Klump says a crow may spend several minutes manufacturing one tool. "Given that they could get a rough tool in a few seconds, they do spend quite some time to modify it," she says. Once the animals have perfected their tools, it seems they really don't want to lose them. Crows in the wild can lose their sticks not only to gravity, but to thieves as well, Klump notes. While putting a stick into a hole seems like the better method for preventing drops, storing it underfoot may keep it safer from other birds. "There is still a lot to explore about crow tool use," Klump says. For example, she's curious whether crows are more careful with tools that they've made themselves, compared to tools that are provided for them. Researchers also don't know much about how crows manage their tools in the long term, over hours or even days. We pants-wearing animals "are just beginning to understand things a bit better," Klump says, while the crows seem to know just what they're doing. Image and video: Klump et al.

Klump BC, van der Wal JE, St Clair JJ, & Rutz C (2015). Context-dependent 'safekeeping' of foraging tools in New Caledonian crows. Proceedings. Biological sciences / The Royal Society, 282 (1808) PMID: 25994674

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