Shorebirds show pronounced inter-sexual and inter-specific variation in bill length and shape as well as in foraging behaviour and are a candidate group to study the diverhology. Certain aspects of gross bill morphology and micro-anatomy are known to be adaptive to specific modes of foraging. ‘Pecking’ is characterised by feeding on intertidal invertebrates at or near the sediment surface (epifaunal prey). ‘Probing’, by contrast, consists of inserting the bill into the sediment, allowing the capture of invertebrates that live below the sediment surface (infaunal prey). Probing is observed more frequently in species with long and curved bills than in species with short and straight bills The difference appears, at least partly, attributable to a probing curved bill being able to ‘inspect’ a greater volume of sediment than a straight bill of equal length

Waders begin to feed as the tide recedes. Shorter-legged waders, such as knot (Calidris canutus) and dunlin {Calidris alpina), feed on the mud as it becomes exposed; whereas birds like redshank (Tringa totanus), curlew {Numenius arquata) and avocet (Recurvirostra avosetta) use their long legs to wade out to feed in shallow water. In the Autumn waders feed only during the day; but with the onset of short Winter days, they have to feed during the night as well.

The knot is one of several species of waders belonging to the genus Calidris. It is also one of a much larger group known as sandpipers. Knots reach Britain in late Summer and Autumn from their breeding grounds north of the Arctic Circle. These short greyish (in Winter) waders are midway between the size of redshank and dunlin. Huge flocks of knots can be seen feeding together on the mud banks of estuaries in Winter, especially in the Dee Estuary (Cheshire), Morecambe Bay, Foulness and the Wash. The proposed land reclamation of these areas would seriously threaten a large proportion of the knots overwintering in Britain. Knots remain closely packed together even as they fly-when one second the flock is dark and the next pale, as they wheel round to show their undersides. When in flight the pale rump and tail show clearly. It is the bill length which determines the depth down to which waders can penetrate the mud to feed. Figure 24 compares the bill length of six waders relative to the burrows of some of the mud-living invertebrates. Knots, which have a bill length of 30-38 mm (ii-il inches), feed on crabs, worms and small molluscs.

The dunlin also a sandpiper-is the smallest common wader. In Winter dunlin are grey-brown above, with a dark rump edged in white. The breast is streaked with grey and the underside is white. In Summer they are easy to spot, when they develop a rust back and a black belly. Some dunlin remain in Britain to breed-on moors and marshes-but large flocks fly off to breed in Iceland. Dunlin have a bill length of 25-34 mm inches) and
they feed on small crustaceans and worms.

The black and white oystercatcher (Haematopus ostralegus, with its long red bill and pink legs, cannot be mistaken for any other bird. Widespread all round the British coast, oystercatchers are especially abundant in the west. The name oystercatcher is misleading, for it does not feed on oysters which live below LWST. Oystercatchers feed between the tide marks on cockles and mussels and are one of the few waders which do not take their food whole. When mussels are covered by water, their shells are open; therefore the oystercatcher can insert its beak to sever the shell muscle so that the shells open out. When the mussel is exposed, the shells are closed, so the oystercatcher has to use a different technique. It smashes the shell with its beak until it succeeds in breaking the bottom end of the lower valve. Very often pairs of mussel shells with a piece of shell broken from one valve, can be found on the shore. An oystercatcher eats approximately its own weight each day in mussel or cockle flesh. In Morecambe Bay the rate of feeding varied from 14-51 cockles per hour, with a total of 214-315 cockles being eaten each day by one bird. During the 1954/5 Winter, the 30,000 oystercatchers counted in one part of Morecambe Bay consumed about 22 per cent of the cockles in that area. About 20,000 pairs of oystercatchers breed in Britain. The nest is merely a scrape in the ground, in which 2-3 eggs are laid amongst shells or pebbles. Oystercatchers will also nest inland on moorland areas, on fields and even on shingle beds beside rivers.

The distinctive 'coorli' call of the curlew, haunts estuaries and salt marsh areas from mid-Summer to early Spring, when the birds move inland to breed on moors, sand-dunes and heathland areas. This long-legged, long-billed bird is Britain's largest wader. It has a greyish-brown body with a white rump. The 100-152 mm (4-6 inch) long downwardly-curving bill enables it to feed on deep-burrowing lugworms, ragworms and bivalves.

Avocets are black-and-white waders with bluish legs and a long black upturned bill. Two hundred years ago these birds used to breed in several places along the east coast. But by draining marshland areas, by collecting eggs to make puddings and by killing the birds themselves for feathers to make into fishing flies, man finally wiped out the last breeding colony by 1825. For more than 100 years, no avocets nested in Britain. Then, after the Second World War, a few pairs began to nest on Minsmere and Havergate Island in the Ore Estuary in Suffolk. Now both these areas are R.S.P.B. reserves and more than 100 pairs of avocets come to nest each Summer on Havergate where a permit is required to visit. The R.S.P.B. has in fact adopted the avocet for its symbol. Avocets come regularly each year as Winter visitors to the Tamar Estuary, Devon. They use their 75-92 mm (3-3! inch) long up-turned bill-by sweeping it backwards and forwards through shallow water -to feed on shrimps.

Unlike oystercatchers, which often feed in flocks, the redshank is a more solitary feeder, and has a very varied diet. The redshank's main food is the amphipod, Corophium volutator; but it also feeds on ragworms, Baltic tellins, and Hydrobia snails.

Other waders which come to feed in estuaries all as Winter feeders include:

  • ringed plover (Charadrius hiaticula), throughout the year;
  • grey plover (Pluvialis squatarola),
  • sanderling (Calidris alba),
  • turnstone (Arenaria interpres).

The following are migrants passing through in Spring and Autumn:

  • whimbrel (Numenius phaeopus)
  • greenshank (Tringa nebularia).