Wood thrush

Appearance

The adult wood thrush is 18 to 21.5 cm long, with a wingspan of 30 to 40 cm and a body mass of 48 to 72 g. Among standard measurements, the wing chord is 9.6 to 11.6 cm, the bill is 1.6 to 2 cm and the tarsus is 2.8 to 3.3 cm. It is distinctly larger than the "Catharus" thrushes with which the species is often sympatric but slightly smaller than the common American robin. The longest known lifespan for a wood thrush in the wild is 8 years, 11 months. The crown, nape, and upper back are cinnamon-brown, while the back wings, and tail are a slightly duller brown. The breast and belly are white with large dark brown spots on the breast, sides, and flanks. It has white eye rings and pink legs. Other brownish thrushes have finer spotting on the breast. The juvenile looks similar to adults, but has additional spots on the back, neck, and wing coverts. The male and female are similar in size and plumage.

Distribution

The wood thrush's breeding range extends from Manitoba, Ontario and Nova Scotia in southern Canada to northern Florida and from the Atlantic coast to the Missouri River and the eastern Great Plains. It migrates to southern Mexico through to Panama in Central America in the winter, mostly in the lowlands along the Atlantic and Pacific coasts. It generally arrives on the U.S. Gulf Coast during the first week of April. Fall migration usually begins in mid-August and continues through mid-September. Migration takes place at night, allowing them to find their direction from the stars and orient themselves by detecting the Earth's magnetic field.

Status

The wood thrush has become a symbol of the decline of Neotropical songbirds of eastern North America, having declined by approximately 50% since 1966. Along with many other species, this thrush faces threats both to its North American breeding grounds and Central American wintering grounds. Forest fragmentation in North American forests has resulted in both increased nest predation and increased cowbird parasitism, significantly reducing their reproductive success. A study by the Cornell Laboratory of Ornithology was the first large-scale analysis that linked acid rain to this thrush's decline. Continued destruction of primary forest in Central America eliminated preferred wood thrush wintering habitats, likely forcing the birds to choose secondary habitats where mortality rates are higher. In spite of this, the wood thrush is still considered to be of least concern.

Behavior

The wood thrush is primarily solitary, but occasionally forms mixed-species flocks in the winter. Its breeding territory ranges from 800 to 8,000 m2 in size, and are used for nesting, gathering nest materials, and foraging. Some wood thrushes also defend a feeding territory in the winter. Territorial interactions are usually settled without physical contact, but in high-intensity encounters or nest defense, physical interactions with the feet or bill have been observed. Defense behaviors in response to nest predators include wing flicks, tail flicks, and raising the crest, sometimes escalating to dives and strikes.

This species has also been observed displaying a behavior known as "anting." Anting occurs when a bird picks up a single ant or group of ants and rubs them on its feathers. The purpose of this behavior is unknown, but it is thought that the birds may be able to acquire defensive secretions from the ants possibly used for some medicinal purposes, or that it simply supplements the birds' own preen oil.

Habitat

The wood thrush prefers deciduous and mixed forests for breeding. It prefers late-successional, upland mesic forests with a moderately-dense shrub layer. Robert I. Bertin found that this thrush favors areas with running water, moist ground, and high understorey cover. The breeding habitat generally includes trees taller than 16 m, a fairly open forest floor, moist soil, and leaf litter, with substrate moisture more important than either canopy cover or access to running water. The wood thrush can breed in habitat patches as small as 0.4 hectares, but it runs the risk of higher predation and nest parasitism. The wood thrush's breeding range has expanded northward, displacing the veery and hermit thrush in some locations. In recent times, as a result of fragmentation of forests, it has been increasingly exposed to
nest parasitism by brown-headed cowbirds, as well as loss of habitat in the winter range.

Reproduction

Wood thrushes are monogamous. Breeding pairs form in mid-April to early-May, and usually last throughout the breeding season. Most thrushes find a new mate each year, and mate guarding and extra-pair copulations have not been observed in this species.

Some male wood thrushes arrive at the breeding grounds several days before the earliest females while other males arrive at the same time as the females, establishing territories ranging in size from 0.08 to 0.8 hectares. The female typically leads silent circular flights 1–1.8 m from the ground, with the male chasing. Six or more flights generally take place in succession. The pairs will perch together and feed each other in between flights. The male begins to sing at dawn and dusk a few days after arriving at breeding grounds. Early in the breeding season, the male sings from high perches in the tallest trees, but as the season progresses, it sings somewhat shorter and less elaborate songs from lower perches. Each day's singing begins and is most intense just before sunrise. The male may sing throughout the day but especially at dusk. The song season is usually over by the end of July.

Typically, the female chooses the nest site and builds the nest. However, there has been some indication that the male is able to influence the selection of the nest site by perching nearby and singing. Usually, though, the female chooses whether or not to accept or reject the nest site suggested by the male. The nest is usually sited in a dense patch of vegetation in a tree or shrub that provides concealment and shade. It is usually made of dead grasses, stems, and leaves, and lined with mud, and placed in a fork at a horizontal branch. The nest is not reused. Usually, two broods are attempted, although three to four separate nests may be built before a pair succeeds. Two to four pale blue eggs are laid at the rate of one per day. The eggs are incubated by the female only for 11 to 14 days, with the average being 13 days. Like all passerines, the chicks are altricial at hatching, mostly naked with closed eyes. The female broods the chicks during the first four days after hatching. Both parents feed the nestlings and remove fecal sacs from the nest. The chicks fledge 12–15 days after hatching, but the parents continue to feed them until they become independent and leave the parents' territory at 21–31 days old.

The young wood thrush is able to begin breeding the next summer. Most females lay their first eggs in mid-May, but older females may begin laying sooner. Pairs usually attempt to rear a second brood no later than late July, with the last of the young fledging around mid-August. About half of all wood thrush pairs successfully raise two broods.

Food

Soil invertebrates and larvae make up most of the wood thrush's omnivorous diet, but it will also eat fruits in the late summer, fall, and late winter. It occasionally feeds on arboreal insects, snails, and small salamanders. The young are fed insects and some fruit. After breeding and before migration, the wood thrush will switch from insects to fruits with high lipid levels. In the summer, low fruit consumption and lipid reserves require the bird to feed on insects continuously in order to meet its metabolic needs.

The wood thrush forages mainly on the forest floor, flipping leaves over with their bills to reveal insects. It can be observed hopping around in leaf litter and on semi-bare ground under the forest canopy. Fruits are swallowed whole.