The sight of huge numbers of migrant knots twisting and turning through the air together in the winter sky must be one of the UK’s most breath-taking natural events.

But this behaviour has mainly been observed during the day. And little is known about knot activity at night.

So a team of programme makers set out to find out how knots behaved under the cover of darkness.

'Wisp of smoke'

“It’s the first time that knots have been filmed flying in these really tight formations at night,” said naturalist and presenter Iolo Williams.

The sequence, recorded at Snettisham RSPB reserve in Norfolk, UK, was filmed for BBC Two’s Winterwatch.

And the team captured the mesmerising moment using the same thermal imaging technology they used to reveal seals' secretive night time behaviour on the north Norfolk coast.

Knots (Calidris canutus) are thought to fly in tight formation in huge flocks to avoid and confuse predators such as peregrine falcons, which hunt them in the day. The programme’s makers had also postulated the birds might be more dispersed because they would be unable to see as well in the dark.

But despite the pitch black and lack of predators, the knots kept their tight formation for the spectacle.

“I was able to, on the [camera] monitor, watch the birds and the display that they put on was every bit as spectacular as when I’ve watched them by day,” said Mr Williams.

“It was like a long wisp of smoke.”

Mr Williams said he was a little surprised to see the flock flying with the same density and agility at night as by day: “You think, well, what’s the point of it at night if you’re not trying to evade predators?

“But then you also think well maybe it’s something that’s ingrained in them by now – maybe it’s in their genes now to fly in this tight formation to avoid being eaten.”

Knots' activity is governed by sea tides: when the tide is out they feed on invertebrates in mud flats. But when the tide rushes back in they are forced to leave in massive numbers, resulting in the spectacle of tens of thousands of knots filling the sky and flying in synchrony.

And when it comes to aerial ability, the knot's migration is among the longest in the animal kingdom. They journey from their Arctic breeding grounds to coasts and estuaries in Europe, Africa and Australia. Large numbers of them can be seen in the UK during winter.

See the entire incredible night time knot display film with Iolo Williams in Winterwatch on BBC Two on Wednesday 21 January at 20:00 GMT.

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