Why is Mud so Good for Shorebirds?
When we think about Northumberland and its stunning coastline, the first image that springs to mind may well be the long stretches of golden sandy beaches that seem to go on forever. However, one habitat that may sometimes get overlooked is one that is one of the most important – mud.
If you visit an estuary at low tide, you are pretty much guaranteed to see mud. Estuaries are one of the most biologically rich environments on the earth, due to the supply of nutrients from both freshwater and marine inputs. A huge variety of creatures live in estuarine habitats, including fish, crabs, worms and tiny marine plankton. All have adapted to cope with the daily changes in salinity and water level brought about by tidal action.
However, the areas that are exposed when the tide recedes (know as the intertidal zone), which are usually stretches of muddy shore, experience very little salinity change, creating a sanctuary for those species living within the sediment as they do not have to cope with the high stress brought about by changes in salinity. Adding to this the large and constant supply of nutrients brought by the mixing of fresh and salt water at high tide, a huge abundance of marine invertebrates thrive in this environment, including worms, crustaceans and bivalves.
However, it is not all plain sailing for these creatures. They have one very serious issue that they need to content with – predation by a hungry shorebird!
It has been calculated that one cubic metre of mud from the River Mersey estuary contains the same number of calories in 16 Mars Bars (The Wildlife Trust – Living Seas North West)! With such a large diversity of food on offer, it is no wonder that the mud flats associated with estuaries are so attractive to shorebirds. The birds come in all sorts of shapes and sizes and have evolved special features to help them to find their preferred food as easily as possible, such as the shape of their bill and the length of their legs. 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 across different habitat types. Please visit our ‘Fits the Bill’ page on our website for more information about the different bill shapes of some of our shorebirds – http://spaceforshorebirds.co.uk/fits-the-bill/. Male and female birds and have slightly different sized bills to reduce intraspecific competition for food. With so many birds congregated in one area, it makes these mud flats a productive hunting ground for birds of prey such as peregrine falcons and sparrowhawks.
Where conditions and the lay of the land allow, a salt marsh can form in or close to an estuary, for example where a section of the river is very slow-moving allowing sediment to build up. The more sediment that is deposited, the height and elevation of the mud flats increase until they are no longer covered by the sea at high tide, allowing vegetation to establish and hence more sediment to be retained. This has occurred in stretches of the Coquet and Aln estuaries. The water that is still flowing through the marsh creates small muddy creeks, which are ideal hunting grounds for shorebirds.
Although most often associated with estuaries, mud flats can form on areas of the coast away from rivers, given that there is a low energy environment to allow fine silt and clay particles to settle. For example, this can occur on a sheltered section of a bay which experiences low wave action. As discussed above, muddy sediment supports a huge density of life, higher than that of a sandy shore. Where there is a large abundance of readily available food, shorebirds will gather, and muddy sections of the coast, such as Boulmer and Newbiggin, can support high numbers of shorebirds.
During their time here in Northumberland, shorebirds must feed as much as possible to keep their energy levels high enough to survive the winter. To reduce energy expenditure when feeding, they want to find food with as little effort as possible, and so will gather in areas which are rich in dining options. If they are disturbed, either by predators, dogs or people, they may fly away and be forced to try and find food in sub-optimal areas.
Next time you are on the coast, take a moment to look at the different sediment types you come across, and try to notice the different shorebird assemblages using each habitat. Muddy and rocky shores generally support a higher abundance of birds, so if possible, it is best to avoid walking on these areas to minimise disturbance to the birds. Thank you.
