Wild ducks divide themselves into two natural groups according to habit and the manner in which they obtain their food. Sportsmen and fowlers refer to those divisions as "surface" and "diving" ducks. Those which comprise the first class feed exclusively upon the surface and inhabit fresh water; the latter are mostly marine forms, and in procuring their food the whole body is submerged. Among the surface-feeding ducks are the shoveller, sheldrake, mallard, pintail, gadwall, garganey, widgeon, and teal; whilst the latter include the tufted duck, scaup, scoter, surf scoter, velvet scoter, pochard, and golden-eye. Other British ducks which would come naturally into one or other of these groups, but are more or less rare, are the eiders, American widgeon, red-crested pochard, smew, the mergansers, and the buffel-headed, long-tailed, ruddy sheld, Steller's western, ferruginous, and harlequin ducks. From the fact of their resorting to inland waters the surface-feeding ducks are perhaps the best known. All of them are shy, wary birds, and as difficult of approach as to bring down. Nearly all the species which inhabit fresh water feed during the night, and fly off to the hills to rest and sleep during the day. All of them are birds of considerable powers of flight, and an interesting fact in their economy is the power of the males to change their summer plumage so as to resemble that of the females. As this adaptation only takes place during the breeding season it is probably done for protective reasons.