Whether you are watching wild ducks or raising them in captivity, it is fairly easy to determine the species in adults but usually much harder in ducklings. With sensitivity to context cues, a good eye and some basic background knowledge about ducks, even amateur birders can identify the species of brown and yellow ducklings.
Mallard ducks are the most common wild duck of North America; they have a yellow face, chest and belly, with a chestnut-brown crown, back, wings and tail. Mallard ducklings have a brown stripe through the eye, and a little brown spot by the ear. Their legs are also brown. Mallard ducklings can be found in city parks, natural rivers and lakes and vernal puddles. They are usually bolder than other species because their parents often dwell close to humans; ducklings in the wild will be more shy.
Wood ducks are another species with offspring that are mostly brown and yellow. Aside from being a slightly darker shade of brown, they look almost identical to mallard ducklings and are most easily distinguished by identifying the parents that accompany them. Because wood ducks are cavity-nesters, you are more likely to see them in remote places rather than city ponds or places where humans dwell. If you see a brood of ducklings in a remote marsh but no parent is present, chances are they're wood ducks -- they tend to be skittish and leave when humans approach.
Like mallards and wood ducks, pintails are a species found throughout North America. The brown and yellow ducklings are best distinguished by identifying the parents. Pintail ducklings have a yellow face and a brown crown and body. Like mallards and wood ducks, they also have a brown stripe through the eye and a brown spot by the ear -- although in pintails, this spot tends to be slightly darker and larger.
Muscovy ducks are commonly spotted around human dwellings. Although they are not native to the U.S., they have successfully established breeding colonies around many public parks. Muscovy ducklings are yellow when first hatched, but become brown and yellow after a few weeks. They have a similar colour pattern to that of the mallard. Muscovy ducklings eventually lose their brown eye stripe and keep only a brown crown; their bill also turns from brown to light pink. Muscovy ducklings often have multicoloured (brown and orange) feet that turn orange as they mature.