RICHARD WILLIAMSON Nature Trails...Mysterious bat birds swoop in to pay us a fleeting visit...

What a strange sight. They do look very much like bats with their black bodies and long grey wings.

They are black terns, on their way from Africa to the marshes of Eastern Europe. Now and then they almost swarm over the city’s ‘Lake District’, the flooded gravel pits.

I first saw them here on May 1, 1965, when 800 passed close to the city.

They catch demoiselles and darters, water beetles and caddis flies.

They do not stay long. Sometimes they pass us just off the coast at Selsey.

Their long fluid wings push like propellers. They are one of the ultimate flying bodies, speed with grace.

They work a lake from end to end in a flock. When they have herded every edible body forward and devoured that fuel they return as one back to the beginning, and start their hunt once more from the windward shore. They are as quick as falcons, pursuing their living food on in panic, snapping bodies and wings in one, fuelling the tight muscles which drove them past the Sahara desert and which must as quickly take them on into the Siberian lakes.

We used to have them, on the Norfolk Broads. But the last one was shot in 1848. Under a glass dome in the drawing rooms of the wealthy, they looked in death as saturnine as spirits from the Styx. There they stood with glass eyes fixed upon their captures as if poised to pursue for ever the fantasies of the nether world.

One can imagine the Celts with their worship of water and their certainties that ancestors returned beneath the waves and ripples into aqueous rebirth thought that black terns might be those ghosts returning.

Some nest as close as Holland, so if you are in the country of the black windmills in summer you will probably see smaller sails revolving.

They perform a lovely courtship, flying very high until almost out of sight in a group of a 20 or 30 birds. Then they divide into smaller groups, the females having an idea by then of who their mates shall be, although this may be play acting as they probably mate for life.

From this height they glide on stiff wings slowly back to the ground, looking like those little paper-card Spitfires we used to play with as children. Down they come to water. He will catch a small fish which, when I have seen them on the polders, always reminds me of a small silver bracelet. But he makes her beg for this and she will mew then like a kitten.

The nest is a soggy pile of riverside weeds, with eggs densely speckled as though with pepper.

One can just imagine the Saxons and before them the Romans, collecting these in Amberley Wildbrooks to roast over their fires of willow twigs and reeds.

Nowadays these mysterious birds are here today and gone tomorrow, bats out of the nell of passing day.