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If you start talking about identifying rare shorebirds with most expert birders, they will stress that the first thing you must do is age the bird. Stray plovers and sandpipers are most often found during southbound migrations (after the breeding season). At this time of year you may encounter two age classes of these waders: adults and juveniles.
Adults or more precisely, after hatch-year (AHY) shorebirds go through two molt sequences during the course of the year. In the late winter/early spring they will undergo a partial prealternate molt that results in a bright "breeding" plumage. Flight and tail feathers are not replaced during the prealternate molt, thus by mid-summer they often look worn and faded when compared to brighter, less worn body, head, and wing covert feathers that were replaced during the prealternate molt. Some shorebird species go through prealternate molt before leaving their wintering grounds, while other species change appearance during their northbound migration (i.e. American Golden Plover).
After nesting duties are completed, a prebasic molt, occurs. During this late summer to early winter molt, adult shorebirds replace all of their feathers and the resulting plumage is typically characterized by dull brown and gray tones. This feather replacement occurs in an orderly fashion such that adult birds are never rendered flightless. The timing of the prebasic molt varies from species to species, but as a rule southbound adults will show some evidence of feather replacement. From July-October the plumage of adult birds is often characterized by a disheveled appearance, with feathers of varied lengths, size, and shape. Adults have rather large and often elongated covert feathers. As their plumage transitions, feathers will not lay together neatly and you can often see loose feathers that are about to fall out, sort of the like a child's dangling loose tooth.
Juveniles or hatch-year (HY) birds acquire a complete set of feathers (juvenile plumage) before they commence their first fall (southbound) migration. Most will retain this juvenile plumage until they arrive on wintering grounds. Upon reaching their wintering grounds, the go through a prebasic molt, which results in an appearance that is virtually identical to that of an adult. Juvenile birds are characterized by a very neat, clean plumage. All the feathers seem to match and lay together nicely, presenting a smooth, sleek appearance. There are no loose feathers and very little color or pattern variation within a particular feather set on hatch-year individuals. Juvenile-plumaged shorebirds often show warmer brown, reddish-brown, or buff in their plumage, where transitioning adults are often duller in color.
Finally, it is important to remember that among long-distance migrant/Arctic-nesting shorebirds, adults leave the breeding grounds before juveniles. Adults are normally on the move by the end of June and peaking in the Lower 48 sometime in mid-to-late July. Juveniles of most species are rarely seen at mid and lower latitudes until the very end of July or early August. With many species, there is little overlap among southbound adult and juvenile shorebirds. From 15 August on the overwhelming majority of shorebirds one sees in the Lower 48 states will be juveniles.
In the past, standard field guides either ignored or barely covered the plumages of juvenile shorebirds, so it was hard to learn how to age these birds. Today, however, nearly any standard field guide will illustrate or include photos of juvenile-plumaged birds. Further, there are some excellent specialized photo guides covering this group of species. Understanding the age-related timing of shorebird migrations and the basics of separating adults from juveniles will go a long way in helping you sort through the expected birds and find a rarity