May 11, 2008
By Susan Benson,CNHM Director of Education
Wood thrush or American redstarts; they might be just one bird species, but they both share two habitats. How amazing that the same individual bird that returns to our yard each spring spent the winter further south. Many neotropical migratory birds breed in North America and spend our “winter” in Central or South America.
Over the years, land in North and South America has changed dramatically due to human land uses. Current human population pressures in Central and South America force people to clear forested land for homes, fuel, and agriculture. In these tropical habitats, bird migrants compete with resident birds for smaller habitat areas. These migratory birds might also be hunted for food or removed as pests of agriculture.
In the U.S. there is more forest cover now than at the turn of the century, but the forests are fragmented into smaller parcels, affecting habitat quality for wildlife that depends on interior forest conditions. Even small roads, running through a forest, fragment it and affect forest interior ground nesters. Nest predators such as brown-headed cowbirds, blue jays, raccoons, and feral cats gain access to songbird nests in these smaller forested areas. Wood thrushes and American redstarts are two birds that although present in the northwoods, can be affected by fragmented forests.
Wood thrushes forage for food on the ground, picking up insects and the occasional berry. Female wood thrushes build their nests in deciduous forests as low as six feet off the ground in tree branches.
Although it is a very common species of Eastern forests, the wood thrush is of high conservation concern because of steady, long-term population declines throughout its range for many reasons. According to the Cornell Lab of Ornithology's Bird Population Studies, this species has declined 43% since 1966. Declines in wood thrush populations have been linked to acid rain and forest fragmentation. This species seems dependent on large tracts of mature forest in some parts of its range, but is tolerant of disturbance in other areas. In a study done by Hoover (1995,) he found a 12% nesting success in a 22 acre forest while there was a 72% success rate in a 300 acre forest. In winter, wood thrushes are vulnerable to deforestation in the lowlands of Central America. The brown-headed cowbird frequently parasitizes wood thrush nests. In some parts of the Midwest all of the Wood Thrush nests contain at least one cowbird egg, and some may contain up to eight.
American redstarts hover while gleaning foliage, flushing out the hiding insects. They move rapidly while foraging, flashing their wings and tail to flush insect prey. They prefer second growth deciduous forests that contain abundant shrubs. Nest sites are located at the crotch of a branch from the main trunk of a tree. Nests are placed from 3-45 feet above the ground. They winter in the southern United States, Mexico, Central and South America. Declines have been seen in some areas, but the species is still widespread and abundant. Cowbird parasitism has been reported at 20% for this species.
While birding around the house this spring, enjoy these bird species. Listen for the flute-like sounds of the wood thrush or the varied musical see notes of the redstart.
Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.