In the lower reaches of the Carson River watershed, the Stillwater National Wildlife Refuge hosts large breeding colonies of white-faced ibis ("Plegadis chihi") and is frequented by non-breeding American white pelicans ("Pelecanus erythrorhynchos"). In winter the refuge supports wintering tundra swans ("Cygnus columbianus") as well as hosts of ducks and geese. The Carson River watershed also provides habitat for many smaller species of birds such as tree swallows ("Tachycineta bicolor") and house wrens ("Troglodytes aedon)." Recent research of these two bird species has shown significant mercury concentrations in the livers and eggshells of the birds nesting along the river.