 Loons and grebes, which nest on lakes but winter at sea, are usually categorized as water birds, not seabirds.Although there are a number of sea ducks in the family Anatidae that are truly marine in the winter, by convention they are usually excluded from the seabird grouping.Many waders (or shorebirds) and herons are also highly marine, living on the sea's edge (coast), but are also not treated as seabirds.