Northwest Europe's threatened Bewick's swan population has been boosted by a bumper year for chicks.
Numbers of the bird have declined dramatically since the 1990s.
Up to 7,000 Bewick's swans usually migrate to the UK, arriving in October and flying back to Russia in March.
But surveys this year show the number of young among these wintering flocks has risen to 17.6%, compared to an average of around 10% over the past 10 years.
Ornithologists have reported an overall average of 14% young swans in flocks across northern Europe, the highest since 2001.
"It really is fantastic to see so many cygnets arriving back. They have certainly been few and far between in recent years," said Julia Newth from the Wildfowl and Wetlands trust (WWT).
Bewick's swans travel 2,500 miles (4,000km) from their breeding grounds within the Arctic tundra in Russia to spend the winter in the warmer British Isles and other parts of northern Europe, such as the Netherlands and Germany.
The smallest swan in Europe, Bewick's swans are distinguishable from fellow migrant whooper swans by their size and small yellow blob on their black beaks, rather than the whooper's yellow wedge.
Experts are still trying to understand what has caused this year's bumper breeding session.
"[It's] the golden question that we don't have the answer to yet," said Ms Newth.
"Weather is thought to be a big factor, but it is not yet fully clear so we can only speculate at this stage."
Ornithologists are also still trying to find out why the northwest Europe Bewick's swan population has been in dramatic decline.
Known dangers to swans include illegal shooting and lead poisoning, according to the WWT.
Swans eat grit to help their digestion. But accidental ingestion of spent lead gunshot on the ground can cause severe poisoning.
Post mortem tests at WWT reserves have found that almost a quarter of dead swans found at the sites had died from lead poisoning, according to the organisation.
Man-made structures such as pylons, wind-turbines and power lines also pose threats to flying swans as the birds' large size makes them unable to manoeuvre quickly in-flight to avoid danger.
But the higher number of cygnets reported across northern Europe this year "will hopefully boost [the swan's] numbers", said Ms Newth.
And the arrival of an adult breeding pair of Bewick's swans with six cygnets in tow at the WWT Slimbridge reserve in Gloucestershire is the largest Bewick's swan family recorded at the site.
"We still need to find out what is driving down Bewick's swan numbers," said Ms Newth.
"But this year's good breeding season is very welcome news."
Source: http://www.bbc.co.uk/nature/20800129
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