All species in the order are highly adapted for an aquatic existence at the water surface. All are web-footed for efficient swimming (although some have subsequently become mainly terrestrial).
It is thought that the Anseriformes originated when two groups of specialised filter feeders diverged from the Charadriiformes (plovers, gulls and allies) by evolving different mechanisims to filter small food particles from water. The flamingos developed a filter structure in the lower part of the bill; the ancestors of the Anseriformes developed the characteristic bill structure that they still share. The combination of the internal shape of the bill and a modified tongue acts as a suction pump to draw water in at the tip of the bill and expel it from the sides and rear; an array of fine filter plates called lamellae traps small particles, which are then licked off and swallowed.
Bird family tree
All Anseriformes have this basic structure, but many have subsequently adopted alternative feeding strategies: geese graze on plants, the saw-billed ducks catch fish; even the screamers, which have bills that seem on first sight more like those of the game birds, still have vestigal lamellae.
The Anseriformes and the Galliformes (pheasants etc) are ancestral to neognathous birds, and should follow ratites and tinamous in bird classification systems.
