Modern birds have highly sophisticated jaws, much more flexible than our rigid skull and mandible. Birds can move their whole upper jaw, or parts of it, relative to their cranium, because of regions of thinner bone called bending zones. One of these forms of flexibility is called distal rhynchokinesis, where a section of the upper beak, near the tip, bends relative to the rest of the beak. Interested to know why long-billed shore birds use this unique form of beak bending, Sora Estrella and JoséMasero filmed wild and captive shorebirds – curlew sandpipers, dunlins,sanderlings and little stints – as they fed on brine shrimp in water(p. 3757).
Analysing films of the birds' feeding habits, Estrella and Masero found that both wild and captive birds used distal rhynchokinesis to capture prey. Captive dunlins used rhynchokinesis in around 90% of prey strikes, where they are aiming for their brine shrimp prey with their beak open, and also 76% of the time when they grabbed prey. Rhynchokinesis helped them transport the shrimp from the tip of the beak and into the mouth 42% of the time, and sped up this process. The birds also used rhynchokinesis more with larger shrimp,and in these cases the beak bent more. The authors suspect that this mechanism allows the birds to feed flexibly and opportunistically, saving time and energy, and may also have contributed to their ability to exploit habitats that other birds might ignore.