Dark clouds are forming overhead this autumn – but it is good news for nature lovers.

At this time of year, huge flocks of starlings are treating us on a nightly basis to one of the UK’s most amazing wildlife spectacles.

Hundreds of thousands of starlings are turning the sky black each evening as they come together in massive clouds, wheeling and swooping in unison.

The show, known as a “murmuration”, can be seen at many sites across the region, including RSPB nature reserves at Strumpshaw Fen in the Broads, Snape in Suffolk and Fen Drayton in Cambridgeshire.

Many of the autumn roosts are already forming well, and more and more birds will flock together as the weeks go on, with numbers swelling to around 100,000 in some places.

The huge gatherings are at their largest in winter, as they are boosted by thousands of migrant birds visiting from the European continent for Britain’s milder Atlantic climate.

Experts do not know for certain why and how the birds behave in this manner, but it is most likely that they do so to protect themselves from predators, each spending as little time as possible at the edge of the flock in order to remain safer.

But despite the incredible size of the flocks, these numbers are just a fraction of what they used to be. Starling numbers have declined by more than 70pc in recent years and the species is now on the critical list of UK birds most at risk.

The decline is believed to be the result of a loss of permanent pasture, a general loss of mixed farming and fewer vital food supplies such as crane-fly at critical times for fledged young.

At Strumpshaw Fen, an estimated 10,000 or more starlings have been seen each evening this week, putting on their amazing display shortly before dusk when they settle together into the reeds for the night.

The birds have been roosting at the reserve for about three weeks now, and are expected to remain until the end of the year, although their numbers are probably at a peak around now.

Tim Strudwick, RSPB site manager at Strumpshaw Fen, said it was the best display of starlings seen at the reserve for about six years.

“The starlings will have been feeding mostly on farmland and grazing marsh in the area,” he said.

“Quite big roosts can build up in the reed bed because it’s a sheltered and warm environment and there’s very little wind chill. They really pack in.”

The birds generally arrive from further down the Yare valley in smaller groups, flocking together and diving and swooping before descending into the reeds.

All the time they are watched by predators including marsh harriers and sparrowhawks, which sometimes attempt to snatch a starling from the flock.

Other predators, including otters and even bitterns, can sometime disturb the birds once they are settled in the reeds, sending them skywards once more.

The nightly displays are proving a real draw to visitors at the reserve. “It’s so easy. You don’t have to walk far and it’s one of the best 45 minutes you could spend to watch wildlife,” said Mr Strudwick.

To find out more, please visit www.rspb.org.uk/reserves