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The whistling, or "tree," ducks comprise a group of eight species of world-wide, though primarily tropical, distribution. A single genus, Dendrocygna, is generally accepted, and indeed the species form such a homogeneous group that it is extremely difficult to establish intrageneric differences upon which to judge probable species relationships. In most respects, whistling ducks are very gooselike, differing from geese mainly in their more specialized tracheal structure, their whistling voices, and their distinctive downy plumage patterns. They possess relatively long legs and large feet, and they can dive remarkably well.