Hidden Britain
Nick Baker on a wader with an extraordinary bill
Estuaries are buzzing with birds in winter, all busy exploiting the riches of this habitat betwixt sea and sand. 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 longbilled 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 beak? 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.
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.
Fantastic foragers
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.
But none of this explains the b ill’s curvature. To understand its si ickle shape, maybe we should ta ake inspiration from a curved b ottle brush, which cleans more of f the inner surface of a bottle th han a straight one. Perhaps by y having a curvature to such a sensitive organ, the curlew maximises m its foraging efficiency. With W a larger sensory surface in co ontact with mud or sand, the b ird gets a wider ‘view’. It can al lso ‘sweep’ a bigger area without co ontinually probing its bill.
Moreover, if the curlew is fo oraging among weed, pebbles or worm w burrows, its inclined bill means m it can reach into more of th hose spaces than straight-billed co ompetitors. Watch curlews fo oraging and you’ll notice that th hey spend a lot of time turning, ti ipping and twisting their heads to o winkle out their prey.
One slight downside to this re emarkable tool is that, because it ts internal space is taken up with w structural reinforcement, th here is less room for a long to ongue. So curlews cannot pull o r pump food up their bill like g odwits. Instead, they must je erk their head back repeatedly to o throw prey items to the back o f their throats.