Each wader has its own set of foraging skills and tools to match, from the mechanical stitching of short-billed dunlin to the deft side-swipe of avocet and the careful probing of long-billed godwits. But one bird stands out: our only wader with a long decurved bill.
Why does the common, or Eurasian, curlew have such a peculiar bill?
A straight piece of hardware makes more mechanical sense. It is best for rapidly penetrating silt to reach prey, is stronger and experiences less stresses and flexion – and thus can be lighter, without the need for extra thickness and internal reinforcement found in a curlew bill. So there has to be another purpose to this species’ bent bill, which affords some other ecological advantage.
What is a bird's bill?
Let’s start with a better understanding of what a bird’s bill actually is. As in many birds, a curlew’s bill isn’t the hard, bony object you might imagine. The bone of the upper part, the maxilla, is attached to the front of the skull with a flexible hinge. This enables the top part of the bill to slip forward. The bill-tip also has the ability to bend, a phenomenon known as rhynchokinesis.
A curlew’s bill is also blessed with extreme sensitivity. Its tip, like in many related waders, is slightly swollen. Inside is a honeycomb of cells, each furnished with receptor organs of two main types: Herbst’s corpuscles sensitive to pressure; and Grandry’s corpuscles, which measure shear forces. Examine the empty skull of a wader and you’ll see the bill-tip is perforated with hundreds of tiny holes – ducting for nerve fibres in the living bird.
How does the curlew's bill work?
In short, if a lugworm, clam or crab so much as emits a bubble of fear, then the curlew will find it. The bird might even be able to go one step further, creating a three-dimensional pressure map. When the curlew pushes its bill into mud or sand, a pressure wave of fluid is forced ahead and to the sides, so any obstacle will distort this effect and be detectable.
Why is the common curlew's bill shaped like it is?
But none of this explains the bill’s curvature. To understand its sickle shape, maybe we should take inspiration from a curved bottle brush, which cleans more of the inner surface of a bottle than a straight one. Perhaps by having a curvature to such a sensitive organ, the curlew maximises its foraging efficiency. With a larger sensory surface in contact with mud or sand, the bird gets a wider ‘view’. It can also ‘sweep’ a bigger area without continually probing its bill.
Moreover, if the curlew is foraging among weed, pebbles or worm burrows, its inclined bill means it can reach into more of those spaces than straight-billed competitors. Watch curlews foraging and you’ll notice that they spend a lot of time turning, tipping and twisting their heads to winkle out their prey.
One slight downside to this remarkable tool is that, because its internal space is taken up with structural reinforcement, there is less room for a long tongue. So curlews cannot pull or pump food up their bill like godwits. Instead, they must jerk their head back repeatedly to throw prey items to the back of their throats.
Female curlews are not just larger than the males, they also have a relatively much longer bill, too. So much so, it is often possible to tell Mr & Mrs apart from a distance as her bill is sometimes around 18 per cent longer than his, and looks much more curved as a result. The reasons for this are not clear, but it may have something to do with reducing resource competition between the sexes.
Main image © Getty Images
