The Crescent Harbor IBA, located three kilometers east of the city of Oak Harbor on Whidbey Island, includes the shoreline and marine waters to the 10-meter depth contour, and the adjacent uplands on the seaplane base at Whidbey Naval Air Station but excluding housing and operations areas. Fifty-five percent of the site is marine foraging area. Marine habitat also includes gravel and rock beaches, tidal channels, salt marsh, and mudflats; adjacent uplands contain open grasslands, fresh water ponds, and mature second-growth conifer forest.
The uplands around Crescent Harbor support the highest density of nesting Northern Harriers in Washington. The marshes, shorelines, and marine waters support moderate concentrations of wintering waterfowl. The shorelines are habitat for an extraordinarily large concentration of one current WatchList species, the Black Oystercatcher; and smaller numbers of two former WatchList species, the Surfbird and Black Turnstone. A total of 105 species have been recorded on this site.