By following Bewick's swans
on their migration and meeting communities along the way, Sacha and an expedition team hoped to see why so many of them were unable to survive the journey.
Mac the Lego Mallard and, above, Benedict the Bewick's swan
, now more widely known as the Tundra Swan, is a large, long-necked, pure white species of waterfowl.
The Bewick's swan
was named after Northumberland-born wildlife author Thomas Bewick, who produced many fine drawings of the bird during his lifetime.
But for other species, particularly Bewick's swan
(but also the long-tailed duck and the velvet scoter), the decline is uniform across northern Europe.
The WWT keeps a record of every Bewick's swan
that comes into to Slimbridge, using variations in the distribution of black and yellow on the bill as unique identifiers for each individual.
Some of the last paintings in the book were by Sir Peter Scott and feature - appropriately enough - the Bewick's swan
Experts believe migratory birds, such as the Bewick's swan
or the whooper swan, which winter in Britain to escape the harsh climes of the Arctic Circle, could be the culprits - and they are on their way back to their breeding grounds in Greenland, Iceland and the Baltic.
These include Bewick's swan
, great bittern, osprey, marsh harrier, corncrake, and the kingfisher.
It is good news for the Bewick's swan
population which has seen steep declines since the 1990s.
The flat, arable land in the shadow of the Dee industrial area has hosted both species of wild swan, with a larger number of whooper than Bewick's swan
, which are still present just inside the Flintshire border.
The Severn Estuary, which closely follows the Dee as the second most important site in Wales, also has internationally important numbers of seven species - mute swan, Bewick's swan
, shelduck, pintail, shoveler, ringed plover and dunlin.
The male Bewick's swan
, which was first tagged back in 1991, is thought to be one of the oldest in its species.
Whooper swans, named after their distinctive trumpeting call, and the smaller Bewick's swan
only visit the UK during the winter when they spend the season here to escape the harsh conditions found further north.
The long-term plan for the reserve includes targets to attract many different birds including bittern, wigeon, shoveler, teal and Bewick's swan
, " explained CCW Gwent Levels Warden, Adam Rowlands.