Population Status Trends
Across the Wood Thrush's entire range, Breeding Bird Survey data from 1966 to 2009 show an average annual decline of 1.9% per year. That means that in 2009 there were only 44% as many Wood Thrushes as there were in 1966. In one portion of the bird's wintering grounds in Veracruz, Mexico, numbers of Wood Thrush in 1985 were estimated to make up only 30% of the species' abundance there in 1960.
Wood Thrush has become a species of conservation concern, and in the process, it has been established as a symbol of the decline of Neotropical songbirds in the forests of eastern North America. This species, along with many others, faces threats on both its North American breeding and Neotropical wintering grounds. Forest fragmentation in North American forests results in increased nest predation and increased cowbird parasitism for Wood Thrushes nesting in fragmented forest, thereby significantly reducing their reproductive success. The continuing destruction of primary forest in Central America eliminates preferred Wood Thrush wintering habitat, most likely forcing birds to attempt to exist in secondary habitats where mortality rates may be higher.
Increased mortality in fragmented forests is often caused by an increase in meso-predators, such as raccoons, skunks, opossums, squirrels, and cats. These predators can be especially abundant in urban areas. Increased forest edge can increase the hunting success of bird-eating raptors as well, especially Cooper’s Hawks.
Forest may look pristine when in fact they are not. In many places in eastern North America, there is an excess of deer. Too many deer can lead to an understory that is too open without enough cover for Wood Thrush. Invasive non-native plants can lower the habitat quality of a forest as well, leading to unsafe breeding sites or lowered food availability for Wood Thrush ands other forest birds.
Wood Thrushes have been shown to be vulnerable to both acid rain and mercury. Acid rain causes a loss of invertebrates that require calcium, such as snails, isopods, millipedes, and earthworms, which are important food sources for Wood Thrushes. Mercury negatively affects both immune functions and brain functions in birds. Pesticide use in forests, including spraying for the control of gypsy moths, can lower food availability for Wood Thrushes and other forest birds.
Many groups have gotten together to form an International Wood Thrush Conservation Alliance. The first activity of the group was a workshop in Veracruz, Mexico, in February of 2010, to study survival of the Wood Thrush on its wintering grounds. The next planned activity is a forest stewardship workshop, probably also in Mexico. The alliance is just getting started.
Conservation activities to benefit Wood Thrush should benefit a variety of forest birds. In rural areas, protecting large blocks of forest is most important. In more urban areas, protecting forest parks from encroachments is crucial. Reducing the impacts of human pets, human-commensal predators, non-native invasive species, and disturbance will help Wood Thrushes and other forest birds.
Wood Thrush is listed as a priority species in Partners in Flight's "Bird Conservation Plan for the Ohio Hills," one of the very few areas where populations of the species have been stable (http://www.partnersinflight.org). One of the objectives of this plan is to ensure the availability of about 2.5 million hectares of deciduous forest to support 725,000 breeding pairs of Wood Thrush.