Birdwatch

The last of the wintering ducks are just about hanging on here, with a handful of wigeon refusing to leave, but otherwise the vast flocks of a couple of months ago have all gone.

In their place are a few pairs of shelduck, female mallards with their broods of ducklings in tow, lapwings and redshanks on their nesting territories and, appearing for the lucky few only, a fine drake garganey.

Hobbies, splendid migrant falcons, are also here in numbers now, the occasional sighting of last month being replaced by regular groups of two or three hunting small insects over the brooks.

Few birds are as incessantly and gracefully on the move as a black tern Chlidonias niger.

Most people think of terns as seabirds and most are. Black terns, however, are a little different. They belong to a trio of species known as marsh terns that breed in large colonies on freshwater wetlands across central, southern and eastern Europe (white-winged and whiskered are the other two).

Unlike the sea terns, they feed mainly on insects rather than fish, and in their feeding habits are more akin to swallows, constantly dipping, swooping and gliding over the water's surface in search of food.

They rarely hover and dive but are able to perform the most remarkable aerial manoeuvres in pursuit of prey. Indeed, many old names for the species allude to its swallow-like appearance and behaviour.

During the 18th century, black terns still bred widely across undrained fens and meres in East Anglia '“ they were apparently so common that in some places people collected their eggs for food '“ but had disappeared by the middle of the 19th century.

In the more distant past, they were presumably common breeding birds in large marshes right across the UK.

Ongoing large-scale wetland re-creation in the Fens may in the future bring breeding black terns back to the UK.

They are now passage migrants in the UK, although they have occasionally attempted to breed in recent years.

They regularly appear in variable numbers at both coastal and inland sites, mostly in southern and eastern England during late April and the first half of May.

Most birds pass along the coast, and on a busy day in early May hundreds can be recorded heading east past coastal watchpoints such as Selsey or Dungeness, although a few dozen is more typical.

Their nearest breeding grounds are in Holland but they may be headed for Denmark, southern Sweden or other sites around the Baltic Sea.

Two black terns spent the afternoon feeding over the reserve last weekend.

Their smart black heads, dark grey breasts, pale grey wings and a striking white undertail made them unmistakeable, and their constant movement up and down over the pools on the Brooks made them easy to pick out.

They did not stay long, though, their urge to migrate sending them on their way by the end of the evening.