By: Gary Zimmer
Though relatively common during the summer breeding season in the Upper Great Lakes Region, the wood thrush is a bird seldom seen but often heard. Somewhat reclusive by nature, its flute-like song concluding with a distinctive trill is easily recognized by experienced bird watchers and can be quickly learned by amateur birders. The males three part song is considered by many as one of the most beautiful songs of all of the birds found in North America. During his song, the male actually sings pairs of notes simultaneously, harmonizing and blending to produce the finished product. Due to its song, the wood thrush has been given several other folk names including song thrush, swamp angel and wood robin. Females are not known to sing but do give a short call when agitated. Both sexes do vocalize at times giving a rapid pit-pit-pit call.
Wood thrush are one of several native species of the Turdidae Family (Thrushes and their close allies) that include the American robin, veery, hermit thrush, Swainson’s thrush, and the Eastern bluebird in our region. They are one of the largest thrushes (7 to 8 ½ inches long), though slightly smaller than it’s robin cousin. The upper part’s of the wood thrush (back of head and upper back) are red-brown in color. The breast and belly are white with large dark, brown spots. This bird has a distinct white eye ring and pink legs. Wood thrush have been recorded living up to nine years in the wild.
Wintering in Central America and southern Mexico, wood thrush begin migrating northward in April, arriving in the Great Lakes Region around the first of May. Most wood thrush pass over the Gulf of Mexico during migration. As with many songbird species, migration occurs at night. This bird primarily breeds in the Eastern US and far southern Canada with the Mississippi River a general western boundary line for this species.
Highest concentrations of wood thrush are found along the Appalachian Mountain range with Michigan and Wisconsin near the northwest edge of its range. However, birders have noted an increase in wood thrush numbers in recent years in the Upper Midwest. It is unclear if this change is due to local habitat changes or a general range shift, though it appears mature forests are better suited for this species.
The preferred breeding habitat of trees (typically deciduous) greater than 50 feet tall with a fairly open forest floor, moist soil and good leaf litter fits many of the hardwood forests of our region. Likewise, our large contiguous forests seem to be favorable for this species. Though it has been found breeding in woodlots as little as one acre, they are much more susceptible to predation and nest parasitism by brown-headed cowbirds on these small tracts.
On the breeding grounds, the wood thrush is relatively solitary, establishing and protecting nest territories up to two acres in size. Much of the birds daylight hours are spent foraging on the ground in leaf litter, flipping leaves over with its bill or moving leaves with its feet. Invertebrates and larvae (caterpillars, ants, crickets, moths, spiders, worms and snails) found in the litter make up most of its summer diet, though they will also eat fruit and berries in late summer of fall. Nestlings are fed insects and fruit by the adults. Prior to migration the wood thrush switches from insects to fruits with higher lipid levels critical for the long migration ahead.
Wood thrush are monogamous with the breeding pair formed early upon arrival in the breeding area. Typically the female selects the nest site and builds the nest. Nest sites are usually located in a dense patch of vegetation in a tree or shrub that provides concealment and shade. Usually made of dead grasses and leaves, the nest is lined with mud and built in the branch fork. A clutch consists of two to four pale blue eggs. Females handle the incubation alone for 11 to 14 days until the eggs hatch.
Born naked and with closed eyes, the young need constant care by the adults. The female broods the chicks in the nest for the first four days. Both adults feed the nestlings and remove fecal material from the nest. If all goes well the chicks will fledge 12 to 15 days after hatching. The parents continue to feed the young for another week or two until the young become independent at three and four weeks of age. After the young go on their own, the adults will often attempt to re-nest, though only about half are successful a second time. Second nests need to hatch by late July in order for the young to be strong enough for the upcoming fall migration. Fall migration begins in mid-August and continues into mid-September.
The majority of the known predators of wood thrush are avian creatures. Hawks and owls are the primary predators of adult birds while blue jays, crows, grackles, great horned owls and sharp shinned hawks often target the eggs or chicks. In urban situations, free-ranging cats are also a serious problem for both young and adults. Snakes, chipmunks, weasels, and some squirrels also will take eggs or the young. As noted earlier, parasitism by brown-headed cowbirds has been reported as a serious nest deterrent in some areas. The cowbirds deposit one or more of their eggs in other birds nests allowing the nest owners to hatch and raise the young cowbirds. The cowbird young grows quickly, often taking up most of the food brought by the unsuspecting parents.
Other mortality factors for this species, as well as many other songbirds, include encounters with automobiles and collisions, especially during migration, with windows, communication towers and wind turbines.
Often without knowing it, loggers and woodland owners have close encounters with wood thrush during the summer months. With modern technology it takes but a few minutes to learn what the special song of wood thrush is or even what this bird looks like. Referring to the wood thrush American naturalist Henry David Thoreau once wrote,
“Whenever a man hears it he is young, and Nature is in her spring; whenever he hears it, it is a new world and a free country, and the gates of heaven are not shut against him.”