Many of the common names given to organisms are descriptive, but sometimes the description is valid for only some of the individuals. Take the Black-bellied Plover (Pluvialis squatarola), for example. Forgetting for a moment that the undertail coverts, which some might take as part of the belly, are snow-white, the males of this species have lustrous black bellies, and in fact almost entirely black underparts.
Female Black-bellied are similar to males but duller above, more brownish in comparison with the males' more spangled black and white look. They also may have some white intermixed with the black of the underparts. Nevertheless, they are clearly black-bellied Black-bellied.
However, this is true for only half the year; the underparts are black only in alternate (breeding) plumage, from April to August or September. For the rest of the year, in basic (nonbreeding) plumage, the underparts of both sexes look entirely white at a distance; brown streaks and bars are apparent at closer range. the upperparts are light brown, with slightly darker markings at this time.
Furthermore, most birds don't get black underparts until they are at least two years old. Juveniles fresh off the Arctic breeding grounds look much like basic-plumaged adults but are a bit darker above, with light markings on the feathers. These birds get increasingly faded and worn during their first winter and spring, then molt into a plumage much like the adults' basic plumage for their second year of life. In their second spring, their plumage is identical to that of the adults.
Even more fun, if confusion can be fun, it takes the birds about a month to molt between these plumages, so there are black-and-white-bellied plovers present during one-sixth of the year. This is mostly in March and September, but not all individuals molt on the same schedule, so these intermediate birds can be present at other times.
One of the most interesting aspects of the plumages of this species is the sexual dimorphism. Males conduct aerial displays on the breeding grounds, so it is understandable why they are black below to be more visible against the sky. But why then should females share the color? They are duller than males, but their plumage change at breeding time is still quite substantial. Perhaps different plumages just function for sex recognition, as they do in so many birds, but then why aren't all shorebirds sexually dimorphic?
Black-bellied Plovers are abundant migrants on the Washington coast and also winter in some numbers, both on the outer coast and in protected estuaries in the Strait of Juan de Fuca and Puget Sound. They are usually in flocks, and an observer with a good spotting scope can study all of these plumages at different times of year.