Unclean birds include those that are: -birds of prey- carrion-eating scavenger birds – ratite birds- web-footed, and zygodactyl-footed birds-waterfowl- flying mammals (bats)

The reason why ducks, geese, and swans are unclean is because they lack some of the characteristics and features of clean birds – they do not have crops, they have different body structures than clean birds do, their body fat is intertwined with their flesh, and they have webbed feet.

Bills
Ducks, geese, and swans have round-tipped bills. Their bills are used for efficient foraging. As waterfowl their diet includes algae, crawfish, frogs, sea plants, and small fish. Clean birds have beaks that are primarily used for eating grains, insects, and seeds.

CROPS – DO NOT HAVE
In a bird’s digestive system, the crop is an expanded, muscular pouch near the gullet or throat. It is a part of the digestive tract, essentially an enlarged part of the esophagus. It is used to temporarily store and pre-digest food [1]. Ducks, geese, and swans do not chew their food and swallow their food whole.

Clean birds are land-based birds, not waterfowl.

WEBBED FEET
Ducks, geese, and swans use their webbed feet as paddles for swimming andfor diving when submerging in water to search for food. Clean birds do not have webbed feet.

THE PELICAN AND SEAGULL – TYPES OF UNCLEAN WATERFOWL
The pelican and the seagull are listed in Scripture among the unclean birds. The pelican and seagull possess webbed feet, as do ducks, geese, and swans. The pelican is a bird that glides upon the water and has a high percentage of body fat intertwined within its flesh to keep it buoyant in water, as do ducks, geese, and swans. The pelican is a similar kind of unclean bird that ducks, geese, and swans are.