Most of the WWT sites have captive collections (Welney and Caerlaverock do not) but all have reserve areas where large hides overlook wetlands frequented by wild birds. The original idea was developed at Slimbridge by Sir Peter Scott, the founder of the Trust. The New Grounds on the English bank of the Severn estuary were an important site for wintering white-fronted geese, which is why the site was chosen. Unfortunately the numbers of these geese that winter in England has declined in recent years (larger numbers stay in continental Europe unless the weather is very bad); I remember seeing Tack Piece covered with several thousand geese in 1974, but I have only seen a few dozen in the last couple of years. There have been similar trends with the Bewick's swans which are attracted to the Rushy Pen by daily feedings of grain.
I think swan numbers have been more reliable at Welney, where they are also provisioned. The pink-footed geese and whooper swans at Martin Mere are doing well and I think this is the best site for bird watching in the winter - I recommend the Ron Barker Hide. I have never visited the barnacle goose flock at Caerlaverock in Scotland, but it's on my 'to do' list. I think the other sites have a range of wild birds, but no spectacularly large flocks.
The latest developments at some centres are wetland areas with canoe trails through them, to let visitors view a range of native wildlife during the summer.
At one time Slimbridge had the largest captive wildfowl collection the world, but as the number of sites has grown each one has developed its specialities, for example Arundel has a naturally clean water supply so it suits sea ducks. Consequently the number of wildfowl species at Slimbridge has been reduced and cranes, waders and a few wetland mammals have been added to the collection.
My favourite WWT exhibit is the Iceland Lake aviary at Arundel, but I'm a sucker for long-tailed ducks
Alan
