In the sky over Gloucestershire: the rare sight of a quarter of a million starlings seeking refuge

It was a common sight once, on winter evenings, the great flock of starlings wheeling in the sunset sky like a living black cloud. Now in most of Britain it has gone.

This spectacular gathering of perhaps a quarter of a million birds, photographed yesterday over the Wildfowl and Wetlands Trust (WWT) reserve at Slimbridge, Gloucestershire, is a reminder of a spectacle that 20 years ago could be seen, not just in the countryside, but in many British cities too.

But the starling, just like that other once-common and familiar urban bird the house sparrow, has seen its numbers crash over the last two decades, and this year was placed on Britain's official "Red List" as a species of conservation concern. And almost without us noticing, the vast flocks which used to descend in a chattering cacophony on city centres to roost, in places like Leicester Square, in London, have melted away.

The Slimbridge flock is almost certainly made up of birds which have migrated to Britain for the winter from Northern Europe and Scandinavia, where the ground gets too hard for them to probe for the daily diet of invertebrates – bugs and grubs and worms – they need to survive.

It is delighting visitors to the reserve, said the WWT Director, Tony Richardson. "We're used to seeing geese and ducks here in large numbers but the starling roost is really quite something," he said. "Most years we get them, but what's variable is the quantity. Last winter it was a smattering, a handful of thousands. This year the sky is black with them."

Most remarkable of all is their response to predators like sparrowhawks, he said, "Sparrowhawks find them extremely desirable, so the starlings all try to be in the middle of the flock. They all go into amazingly tight bunches – they look like black balls. The movement, all turning back and flying in unison when they're probably almost wing-touching, is quite extraordinary."

Defence against predators is probably one of the reasons starlings gather in such numbers in the first place, said the world authority on the bird, Christopher Feare. Other likely reasons include keeping warm at night, and the ability to find good food sites together the next morning.

It is probably the disappearance of their food sources which has led to their decline in Britain, said Professor Feare, who spent years studying starlings as a senior official of the former Ministry of Agriculture. "Starlings need a daily diet of invertebrates and they find them in grassland," he said. "But with the intensification of agriculture in recent years, much grassland has been lost."

British starling numbers are believed to have dropped by more than 60 per cent in the last 20 years, so the great sky-filling flock at Slimbridge will be a rarer and rarer sight. We should enjoy it, Professor Feare said. "It's one of the most wonderful sights in ornithology... one of the great spectacles of nature."