New Caledonian crows, Corvus moneduloides, have unusually stout, blunt, and straight bills. They’re also one of a few species that make and use tools year-round and throughout their entire range. According to a new Scientific Reports study, their bills became specialized over time for manipulating stick tools, which help them fish out juicy grubs.
In humans, certain adaptations have enhanced our ability to manipulate tools. For example, our shoulders, which allow for high-speed throwing, likely evolved in Homo erectus a couple million years ago. This sort of adaptive specialization may have also evolved in New Caledonian crows. They use the tip of a tool – sticks, barbed leaves, or even hooked twigs – to irritate longhorn beetle grubs hiding in tree trunks by poking around their face. When a grub reacts, the clever crow positions the tip of the tool at the jaws, ready to extract the grub when it bites down.
Using shape analyses and CT scanning, a team led by University of Auckland’s Gavin Hunt and Ei-Ichi Izawa of Keio University examined the shape and internal structure of the New Caledonian crow’s bill. Then they compared 11 different landmarks of the bill (pictured to the right) with that of nine other crow species and one woodpecker that has a similar foraging strategy.
Compared to other birds, the upper mandible of the New Caledonian crow is deep and short with a straight cutting edge, and their lower mandible is upturned – a combination that offers strong precision gripping and visually-guided manipulation (pictured below). "The lower mandible actually curves slightly up, which likely gives it the strength it needs to hold the tool," study co-author Kevin McGowan of Cornell said in a statement. "And because the bill doesn't curve downward it brings the tool into the narrow range of the bird's binocular vision so it can better see what it is doing."
Once the crows began using tools, the bill became specialized for handling them. These behaviors likely evolved over a very long period of time, but exactly why the crows started using tools in the first place remains unknown. After all, most birds snatch prey just fine using only their beaks and feet. Toolmaking among crows may have happened by chance, and then tool use become ingrained in their biology.
Images in the text: H. Matsui et al., Scientific Reports 2016
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