“Food, glorious food! Worms, shellfish and sand flies!”
Sound delicious? Well our shorebirds sure think so! Most wading birds are carnivorous, in that their staple diet is made up of juicy worms, flies, bivalves and small crustaceans. Over the years different shorebirds have evolved special features which help them to find their preferred food as easily as possible. Time spent unsuccessfully foraging is wasting precious energy – energy which they need to help them survive.
In our previous blog post, we talked about the rocky shore and the types of marine creatures that live there. Not only do they provide a vast diversity of food sources, but they are also relatively disturbance free, and are therefore a refuge for resting shorebirds, seabirds and sometimes seals too. The majority of the land on the Northumberland coast which lies below the high tide mark is designated as a Site of Special Scientific Interest (SSSI) to protect the nationally significant bird populations found there. Additionally, most areas of rocky shore in the county are further protected as Special Protection Areas (SPAs) due to the number of internationally important bird populations relying on them all year round. Northumberland truly is an important part of the world for shorebirds!
The bill shape of a shorebird governs the food which is available to them. From long, thin and curved to short and pointy, no two species have exactly the same bill shape, allowing them to exploit different food sources. To minimise competition for food between individuals of the same species, females usually have slightly longer bills than the males, meaning there is plenty to go round for everyone! Some species can even change the shape of their bill depending on what type of food they eat, due to their bills continuously growing. This is most apparent for oystercatchers. Feeding on worms within soft sediment will produce a more slender, pointier bill and those feeding on shellfish will develop a shorter flat-tipped bill, allowing them to hammer through the hard shells of their prey.
Many of our shorebirds use visual techniques when searching for food. Some, such as turnstones, are scavengers and use their short thick bill to bulldoze their way through seaweed in search of small crustaceans and other unsuspecting marine creatures. They will also scavenge the washed–up rotting seaweed high up on the beach searching for sandflies and their larvae. Sanderling are also visual foragers – they can be seen in groups running back and forth at the waters edge picking up small food particles deposited by the waves. Other species, such as knot, feed using touch – they generate pressure waves in the water in the sediment and, using special sensors in the tip of their bills, can detect where their unsuspecting prey item is from the rebound of the pressure wave.
Rockpools are a great place to find a tasty meal. Shrimp, crabs and fish are all on offer for longer-legged birds who can wade into the water to find them. Curlews have the perfect bill shape to catch crabs as the downward curve helps them to reach right under rocks and in crevices to find their food. Crabs are one of their favourites, and they tackle this meal by fist pulling off their legs and claws and then gobbling the body down whole!
Photo by Andrew Bottamley
On the margins of where rock meets sea, purple sandpiper can be seen excavating the small cracks in the rocks for tiny marine creatures which have been washed up by the waves. It can be extremely tough for these little waders, as they must endure the wild conditions that the North Sea throws at them just to find a meal. For this reason they are our true rocky shore experts.
However the rocky shore isn’t the only place where our shorebirds go in search of food. Muddy sediment, like that found at Boulmer and Newbiggin, is bursting with life, and the soft sediment means shorebirds which using probing as a foraging technique have easy access to this food supply. Curlew, redshank and oystercatcher can use their long bills to pull out worms buried deep in the mud, and in the case of bar-tailed godwits, they often feed in the sediment that is submerged by the sea! Curlew have an added advantage of having a curve in their bill, as this increases the surface area over which they can search for food. But for smaller waders, their shorter bill means they can’t probe as deep into the sediment. So how can they get to the juicy worms living within the mud?
Ringed plovers have mastered a very cleaver technique to make the worms come to them! They rapidly vibrate their foot, causing the surrounding sediment to almost liquify, resulting in any prey items nearby simply floating to the surface! This is known as foot trembling – the shaking is so quick that you can hardly tell they are moving at all!
The length of time that shorebirds can spend foraging for food is determined by two main factors – daylight and the tide. In winter, daylight hours are short, so birds, especially those which use visual techniques, must spend as much time as possible feeding. A high tide in the middle of the day can reduce foraging time even further, and during this time birds must conserve their energy by resting. In our next blog, we will talk about what our shorebirds do at high tide, and the pressures they face at such a vulnerable time.
