By By Colby Galliher
If you’re looking for warblers on a walk in the summer woods, your first instinct might be to look toward the canopy. But two closely related warbler species forgo those elevated environs for the eddies and banks of forested streams and wetlands. These specialists of sylvan waters are a treat for any birder – and offer a challenging exercise in avian identification.
The northern waterthrush (Parkesia noveboracensis) and Louisiana waterthrush (Parkesia motacilla) are mid-sized, vocal warblers that breed in northern New England. Both forage in still and moving water.
“Waterthrushes are unusual for warblers in the way that they associate with rivers and streams, often feeding right at the water’s edge, or on rocks and logs in the water,” said Chris Elphick, a conservation biologist at University of Connecticut who specializes in wetland birds and ecosystems.
The two waterthrush species share many behavioral and physical characteristics. They occupy similar habitats, where they feed on aquatic invertebrates, small fish, snails, and even salamanders. They also sport nearly identical coloring and plumage, with brown backs, white-to-yellow throats and bellies, and dark streaking on the breast and underparts. Both Louisiana and northern waterthrushes have relatively long legs and bob and sway their rear ends when standing and hunting. The Cornell Lab of Ornithology points out that this tendency “might either help them avoid scaring off their prey or possibly startle their prey into motion.”
These similarities can make it difficult to tell the two waterthrush species apart. Luckily, there are several distinctions between these birds that can help the discerning observer decide which is which. The first is their vocalizations, which are hard to miss as they resound through the understory near forested wetlands.
“The songs are different and probably the best way to tell them apart,” said Elphick. The northern’s song is faster and contains more whistles and slurs than the Louisiana’s, which begins with several sharp notes and ends in a fast twitter. Elphick posited that waterthrushes’ songs are especially loud because these birds “frequently occur near rushing water and need to be heard above the noise.”
The second means of parsing these two warblers is their preferred habitat, but this method has its perils. The Louisiana waterthrush favors moving water. These birds stilt along the mosses, logs, and stones of streambeds, plucking their prey from the current. Breeding males establish territories by patrolling streams and delimiting their domains with boisterous song. Northern waterthrushes, meanwhile, tend to stick to still water. They skulk among the rushes and greenery of wooded bogs and swamps. But, in a wrinkle for identification, northern waterthrushes will also frequent streams. (Case in point: on a recent hike through a forested preserve that features a flowing creek and several swampy areas, I found northern waterthrushes hunting in both the creek and the boggy sections.)
The third and final key to distinguishing these water-loving birds is their plumage, though this is also tricky. “The two species look a lot alike and telling them apart physically is hard,” said Samuel Merker, a research scientist in ecology and evolutionary biology at University of Connecticut. Merker and Elphick both highlighted noteworthy nuances between the warblers: northern waterthrushes have heavier, darker streaking on their breasts and underparts, yellower bellies, and narrower and paler white eye-stripes than Louisiana waterthrushes.
Author and illustrator David Sibley offers additional guidance on his website: “A quick judgment of the ground color of the breast and eyebrow stripe will separate most waterthrushes: bright white on Louisiana, yellowish on northern. A yellowish waterthrush is definitely a northern, while a whitish bird could be either species.” He also suggests, “If you encounter a confusing individual, pay special attention to the width of the eyebrow stripe, and the pattern and extent of streaking on the breast and flanks. Many other features, such as bill size, can offer supporting clues for experienced birders.”
Unlike many warbler and other songbird species, both Louisiana and northern waterthrushes have seen their populations increase in recent decades. This may be thanks to improvements over the last half-century in eastern North America’s water quality, as cleaner, healthier waterbodies promote higher numbers of the aquatic invertebrates waterthrushes eat. Still, deforestation, pollution, and window-strikes all threaten waterthrushes’ numbers.
In your search for warblers, remember to lower your eyes from the canopy when ambling by woodland streams and swamps. Whether by their rich songs or charming behaviors, the northern and Louisiana waterthrushes will reward your attention and help to hone your bird ID skills.
Colby Galliher is a writer who calls the woods, meadows, and rivers of New England home. To learn more about his work, visit colbygalliher.com. Illustration by Adelaide Murphy Tyrol. The Outside Story is assigned and edited by Northern Woodlands magazine and sponsored by the Wellborn Ecology Fund of the New Hampshire Charitable Foundation: nhcf.org.
