Wood Thrush

The Wood Thrush is another accomplished singer of New Hampshire’s hardwood forests. Like all songbirds they have a specialized voice box (the syrinx) that allows them to produce two different sounds at the same time. This allows for some very complicated songs that the human ear can’t really separate into all their component parts. The third portion of the Wood Thrush song even has ventriloqual properties which make it sound like it’s coming from a different location. Most singing is at dusk and dawn, especially after the breeding season winds down in August. Another distinctive vocalization is a series of 3-5 “whut” or “whit” calls. This is often uttered when a bird is agitated and at dusk, and a “whut whut whut” heard late in a September evening may be the only indication that the local Wood Thrush hasn’t yet left on migration.

In the larger scheme of things, the Wood Thrush is a relatively recent colonist of New Hampshire. Prior to the 1900s it was extremely rare in the state, but gradually expanded through the 1960s. During this time, it also expanded into the Great Lakes area and parts of southern Canada. Since roughly 1970 however, its populations east of the Appalachians have been declining, in contrast to stable or increasing numbers to the west. Unfortunately, Wood Thrush densities have always been highest in this eastern portion of its range, meaning that increases to the west have not been able to compensate for the declines.

In part because of these well-documented declines, the Wood Thrush is one of the most intensively studied forest birds in eastern North America, particularly with respect to its breeding and habitat ecology. Studies in the 1970s and 1980s documented how thrushes disappeared from small forest fragments or produced fewer young there because of increased parasitism by Brown-headed Cowbirds. More recent work has implicated acid rain in the decline. Where acid deposition is higher, less calcium remains in the soil and is not available to insects and other invertebrates. This in turn reduces calcium intake by thrushes, which might have implications for eggshell production. The Wood Thrush was also one of the first long-distance migrants to be studied in its winter range in Mexico and Central America, where deforestation was suspected to force birds into less suitable habitats and be less likely to survive until spring.