I and a couple of friends saw this female Aythya at Stege slambassiner at the Danish island of Moen last 3rd of February '2007. At first glance it has similarities with Common pochard x Tufted duck but several features is not right. The bird has a lighter more grey brown colour and the difference in the back and flank colour is not as obvious. The size of the bird is intermediate between Tufted and Pochard. The head shape also differs by not being so edgy. The bird flew when we was waching it and showed no white in the wings.
If this bird is a hybrid involving Pochard and Tufted duck, it is probably not an F1 hybrid but rather (Pochard x Tufted) x Pochard. But it might also be a hybrid between Pochard and some other aythya eg.Ring-Necked Duck as it has some similarities to a bird, Canvasback x RND (number 182) in the Gillhams book: HYBRID DUCKS, THE 5th CONTRIBUTION TOWARDS AN INVENTORY.