Every winter the UK is invaded by wild swans. These raiders come from the North, and unlike the snowfall their arrival proceeds, they are far from silent. Our meres, marshes and farmland harbour wild swans — thousands of them. These are not the semi-tame birds that squabble for scraps in local parks, these are natives of the tundra; their bugling, whooping calls immediately evocative of the harsh wilderness they have left behind.

The impressively noisy Whooper swan and the smaller, rarer Bewick’s are tough birds, but they are still not tough enough to cope with the permanent dark and unremitting chill of an Arctic winter.

Some 16,000 Whoopers, (more than half the entire population), and 7,000 Bewick’s swans (a third of the world’s total number) see out the worst of the cold in the comparatively balmy UK.

It was once believed that swans represented the souls of the dead, and there is something undeniably eerie about the site and sound of a wild flock glowing white in the twilight of a winter saltmarsh.

On first glance, the two species look identical — both sporting dark feet and yellow and black bills. But there are obvious differences.

The Whooper is bigger, only slightly shorter than our ubiquitous Mute swan. It has a thinner, more elegant neck than the Bewick’s, often with an obvious kink at the base. But the big difference is the bill. The Whooper’s yellow markings extend beyond the nostrils — they don’t even get close on the Bewick’s.

The term swansong is thought to come from the ancient belief that swans let out a plaintive note as they died, but in life, both species are not shy about making a racket.

The Whooper earns its name from its distinctive call, often issued in three or four blasts of “hoo, hoo, hoo”. The call of the Bewick’s is higher, reputedly sounding like an excited dog!

The majority of our overwintering Whooper swans come from Iceland. They hunker down in the northern reaches of the UK, with a few from Scandinavia touching down on England’s east coast.

The Bewick’s, named after a Victorian illustrator, is an altogether more elusive beast. Breeding in Russia, the birds congregate on a handful of sites, generally further south than the Whooper.

It is impossible to consider our wild swans without considering the efforts of one wildlife organisation in particular.

Wildfowl and Wetlands Trust (WWT) reserves hold a huge percentage of our swan populations and it was work at WWT’s Slimbridge headquarters in Gloucestershire that paved the way for how we understand their needs today.

WWT swan expert Julia Newth explains: “The Bewick’s were first attracted to the reserve through the efforts of founder Sir Peter Scott. He built a specially designed lake for the swans which enabled them to winter here safely. It provided a predator-free roost site, supplementary food and good grazing on the reserve. He attracted the first birds in the 1950s using some decoy Whistling swans which have a very similar call to the Bewick’s. The Bewick’s are very site faithful, and numbers steadily grew.

“By the late 1970s, there were 400 swans or so visiting the reserve each winter.”

Scott realised each bird had a unique bill pattern. Identifying individuals helped experts to study family behaviour and survival needs of the birds in a far more specific way than previously possible. They found out that the birds showed great site fidelity — more than half of the swans recorded had visited Slimbridge before. The birds formed great partnerships — they mate for life. The longest partnership ever recorded was between Limonia and Laburnum who visited Slimbridge together for 21 years.

Over the years some birds became firm favourites and high on that list was Crinkly, whose deformed neck defined her name.

Her annual arrival prompted stories in the national press, but she has not been seen for the last three years, with staff hoping that she has just switched wintering sites after finding a mate.

And it is not only Crinkly that needs the WWT’s protection. The pressure on Whooper and particularly Bewick’s swans is mounting all the time. Climate change, habitat destruction and illegal shooting all loom large over their continued survival.