Nearly all these waterfowl, though seen on dry land to much the same disadvantage as fish out of water, are exceedingly graceful in either air or water, though not all ducks are as capable of diving as the name would imply. The proverbial futility of a wild goose chase recognises the pace of these birds on the wing, which, though, in common with that of some other birds, popularly exaggerated, is considerably faster than, owing to their short wings and heavy build, might appear to the careless observer. Ducks have a curious habit of adding down to the nest after the eggs are laid and before incubation, and this provision of warm packing is turned to account in Iceland and other breeding places of the eider duck, commercially the most valuable of all ducks. The nest is robbed of this down once before the eggs hatch out, with the result that the female plucks another store from her own breast, supplemented if necessary from the body of the drake. The sitting bird is then left in peace till the nest has fulfilled its purpose, when the remaining down is likewise removed. This down, which combines warmth and lightness, gives a high market value to the eider, which, throughout Scandinavian countries is strictly protected by law and even more effectually by public opinion. The majority of ornamental ducks interbreed freely in captivity. Those who, apparently on reliable evidence, distinguish between the polygamous habit in tame ducks and the constancy of the mallard and other wild kinds to a single mate have hastily assumed that such hybrids are unknown in the natural state. This, however, is incorrect, as there have been authentic cases of crosses between mallard and teal, pochard and scaup and other species, such hybrids having at different times been erroneously accepted as distinct species and named accordingly. The wild duck's nest is usually placed on the ground in some sheltered spot close to still or running water, and the ducklings swim like corks, soon learning the proper use of their flat little bills in gobbling up floating insects and other waterlogged food. Occasionally ducks nest in trees and they have been known to take possession of a deserted rook's nest. There has been some discussion as to whether, in this case, the mother conveys her ducklings to the water in her bill, but this has not actually been witnessed. In cases where, as is often observed, the nest overhangs the water, it has been suggested that the young birds may simply be pushed over the edge and allowed to parachute down to the surface, as they might easily do without risk.