Nature’s Top 40: Britain’s Best Wildlife. Chris Packham

WHEN

       August to November, although waders are present all winter

       WHERE

       The Wash; Dee Estuary; Morecambe Bay; Thames Estuary; Exe Estuary

       The retreating tide reveals a smorgasbord of food for the hungry knot.

      Mike Lane

      It has been estimated that the 155 estuaries around the British coast may well comprise close to a third of the intertidal mud and salt marsh represented in the whole of Europe. These intertidal estuaries are so vital to overwintering waders that they are thought to hold a staggering 1.7 million waders during the winter months, with many more using Britain’s muddy coasts as a refuelling pit stop before travelling on to overwinter elsewhere. This huge figure includes internationally important numbers of knot, dunlin, oystercatcher, redshank, curlew, bar- and black-tailed godwits and grey plover. Many of these species have originated from breeding areas as far away as northern Europe, Siberia, Iceland, Greenland and northeast Canada.

      It seems that Britain has become such a ‘winter wader wonderland’ because of our combination of relatively mild winters and large tidal ranges, which ensure that extensive areas of intertidal mudflats become exposed for feeding on a daily basis. Mudflats become formed in estuaries when the discharging rivers slow to such a pace that they lack the energy to transport their cargo of silt any further, causing it to be dumped. In certain estuaries around the British coast, mud has been continually washed down and deposited since the end of the last glaciation, leading it to reach depths of as much as 30 metres. But, irrespective of the depth of the sediment, it is only the surface layers, which are in contact with light, water and air, that support the chain of life from bacteria to algae, and from worms to shellfish, that are ultimately exploited by the birds.

      All mudflats undergo the experience of tidal inundation twice every 24 hours as the tide rises and falls, and it is this tidal rhythm that governs the behaviour of all the creatures that live either in or on the mud. The intertidal zone is a harsh place in which to live, as the coming and going of the seawater creates an environment of constant change, resulting in hugely varying temperatures and salinities. Many plants also find it difficult to establish a foothold on the fine shifting sediments; this results in much of the plant material that occurs on the intertidal mud, such as diatoms and algae, being of a microscopic size. These factors also mean that relatively few invertebrates have been able to make a permanent home in the mud compared to those in more stable habitats. But those that do manage to become specialised enough to cope can reach vast numbers because of a lack of competition for the organic material available and the huge amount of substrate (mud) available in which to live and from which to feed.

      In addition to mud being easy to burrow into, its structure of fine particles enables the construction of temporary or permanent burrows for lugworms and ragworms, and a variety of shells, such as Baltic tellins and common cockles, in which they can conceal themselves while probing above the surface into the tidal water to feed with specialised siphons and filters. Other creatures, such as Hydrobia snails and Corophium sandhoppers, also emerge from the mud at certain times to graze on the organic debris at the surface; they do so in such profusion that thousands may be present in each square metre of mud.

      The best way to appreciate the vast numbers of living creatures hidden in the mud is to watch the multitude of waders that descend to feed on them. These wader flocks will often contain a whole range of species feeding side by side with little or no aggression, as their techniques for foraging are so variable that they will rarely come into competition with each other. As the wader species have no objection to flocking together while feeding, they are able to gain all the benefits of being in a crowd, such as greater security from predation and the ability to exploit patchily distributed food with greater efficiency by noting ‘feeding hotspots’ where other waders are feeding successfully.

       With no mud exposed at high tide, the knot and oystercatchers avail themselves of the chance to catch upon fortywinks.

      Simon Booth

       The diminutive dunlin contemplating where to probe its bill next.

      Mike Lane

       A feeding bar-tailed godwit – with worm.

      Roger Tidman

      Waders are thought to find food either by touch, sight, or, in some species, by a combination of the two senses. A number of species take advantage of the fact that invertebrates become more active as they are covered by the advancing tide, so there is often a concentration of birds along the tide edge. Knot feeding here are believed to forage primarily by touch as their bills seek out the microscopic snails and bivalves just below the surface, while godwits visually search either below water or at the tidal edge for the recent casts of lugworms before using their long sensitive bill both to feel for, and extract, the worm. In addition to the birds working the tideline, a number of waders have become specialised at feeding in shallow standing water; these include avocets, which swish their scything bills underwater, and spotted redshank, which upend themselves in a manner more reminiscent of dabbling ducks than waders.

      Ahead of the advancing tide, or behind one that is receding, many birds prefer to feed in damp mud because it is easier to probe. The majority of sandpipers, such as redshank or dunlin, feed by scanning the mud to either side of them in walking transects, and, when potential prey is spotted, capturing it either by a peck, if it is a small Corophium snail, for example, or by probing more deeply if it is a buried clamworm such as Nereis. Short-billed birds like grey plovers and lapwings do not tend to use the ‘touchy feely’ techniques of the sandpipers, and rely more on their excellent sight to scan around them for signs of activity while they remain rooted to the spot. When they catch sight of a prey item worthy of capture, they quickly sprint to the spot before it burrows out of their depth. This technique has the advantage that the mud is less disturbed and so does not form a localised ‘prey depression’, which will often happen when the sandpipers are feeding like a miniature herd of grazing wildebeest. When prey activity is low, however, the plovers are not able to begin feeding by touch like the sandpipers, in which case they can resort to a different foraging tactic known as ‘foot trembling’. This technique involves placing one foot on the sediment slightly ahead of the other and rapidly vibrating it up and down, which, it is thought, is meant to simulate the approaching tide, causing their prey to come prematurely to the surface to feed, thereby fatally revealing their location.

      Bill length is, of course, very important for determining which birds are able to reach various prey items at different depths, with the curlews’ long sensitive bill being the obvious tool that equips them well to winkle out even the most deeply buried lugworms and ragworms. While birds like dunlin and sanderling are not able to penetrate the mud to the depths of which the curlew is capable, it is thought that they may well possess the ability to use sensors in their bills either to smell or taste the location of prey hotspots.

       ‘Birds of a feather flock together’, particularly if you are a knot at high tide.

      Roger Tidman

      For most species of intertidal foragers, the usual peaks of feeding activity are on the ebbing and flowing tides, with something of a lull around low tide when many of the birds will roost for a while close to their feeding grounds. Feeding time for these birds is governed strictly by the availability of mud so, irrespective of whether it is day or night, they must feed when the tide is out. While