The Northern Waterthrush is a bird of the northern forests, adding its loud, ringing song to the wooded swamps, bogs, and banks of North America's great rivers and lake shores. Those familiar with this species on its breeding grounds know it for its spirited demeanor characterized by sharp chip calls, emphatic song, and energetic movements. It is also appreciated for its effervescent evening flight song, which compliments those of thrushes and the winnowing of snipes in the boreal forest evensong. However those who know the Northern Waterthrush well also know how seamlessly it switches to its alter ego—a furtive, skulking bird of thickets and shadowy understories. This is the personality familiar to many North American birdwatchers, who see it only on migration in back yards, city parks, and wet places, as it migrates to and from its wintering grounds in the tropical mangroves of Central and South America.
This is a large wood warbler, not a thrush, rarely seen far from shorelines or forested wetlands. Like its close relative the Louisiana Waterthrush (Parkesia motacilla), it continually bobs its body and wags its tail—a key to identification. It is smaller in body and bill than the Louisiana Waterthrush, with subtle differences in plumage, leg color, and song. Nevertheless, some historical data on Northern and Louisiana waterthrushes must be questioned because the two species were confused. Alexander Wilson, for example, and others of his time, failed to recognize them as distinct species.
In the northeastern United States, where Northern and Louisiana waterthrushes often overlap in breeding range and habitat, the Northern Waterthrush behaves aggressively toward other conspecifics but not toward the Louisiana. These two species have evolved differences in foraging and other behaviors that help to separate them; no hybrids are known between them.
Across Canada and Alaska, the stronghold of the Northern Waterthrush, its habitat appears fairly secure, at least compared with that of many other warblers. On its main wintering range, by contrast, its habitat is increasingly threatened. Mangrove (Rhizophora sp.) forests of the West Indies, Mexico, Central America, and northern South America are being cut as burgeoning human populations demand fuel, food, and space, and may also be at risk due to climate change.