Waterfowl are wild birds that live on or near water and swim — ducks, geese and swans above all. The word belongs to a slightly formal, outdoor register: nature documentaries, birdwatching guides, wetland conservation reports, and hunting rules and seasons. A friend pointing at a pond would say 'look at the ducks'; a sign at a nature reserve says 'please do not disturb the waterfowl'.

Two details matter. First, the word means the wild swimming birds specifically — it does not stretch to every bird that happens to be standing in water. Herons, egrets and storks wade on long legs but do not swim, and birdwatchers put them in a separate group called `waders` or `wading birds`. Second, `waterfowl` also has a farming and food sense: in agriculture, ducks and geese together are called waterfowl, as opposed to `poultry` in the narrower sense of chickens and turkeys. That is the sense you meet in the phrase you may have seen in the news — avian flu spreading among wild waterfowl.

The grammar catches people out. `Waterfowl` is normally the same in singular and plural, like `sheep` or `fish`: one waterfowl, many waterfowl. You will occasionally see `waterfowls`, but the unchanged form is far safer.