Wood-warblers are migratory birds. They move seasonally between temperate regions of the Americas, where they breed, and tropical areas, where they spend the winter. The main reasons for this migration are that the insects they feed on cannot be found in great enough quantities for survival during the winter in the northern lands and that the amount of daylight in this season does not allow them time to forage enough daily to survive. They return to the northern regions annually for the large spaces and ample food supplies they require for nesting territories.
Wood-warblers depend on forests. Forests in both temperate and tropical climates have been greatly reduced and altered from their natural state. Forest fragmentation results when forests are cleared by humans and re-duced to small, isolated woodlots (fragments). Birds that require large tracts of contiguous forests may be unable to find suitable habitat to survive. Small woodlots allow many predators and nest parasites to have access to forest-interior bird species. In addition, much of this land is converted to long-term projects such as urban development, subdivisions, farms or other uses. Forest fragmentation and clearing also occur in the tropics as more space is needed for homes, agriculture and development. Loss of habitat along the migratory route can prohibit birds from obtaining the food and rest they need.
Pesticides and pollution can affect wood-warblers and their food sources. Many natural and humanmade obstacles cause problems for these birds during migration. Storms and strong winds can blow birds off course or provide headwinds that slow progress.
As of 2020, the Illinois Endangered Species Protection Board lists one wood-warbler species as endangered in the state and one species as threatened.
Endangered: Swainson’s warbler (Limnothlypis swainsonii) has always been uncommon in the state and is now extremely rare. Preservation of large tracts of bottomland forest and dense cane stands are critical for its survival in Illinois.
Threatened: The cerulean warbler (Setophaga cerulea) prefers to live in forests of more than 1,000 acres in size and is rarely found in forests smaller than 200 acres. This species was once abundant in the state, but loss of large tracts of forested habitat has been detrimental to it. Conservation actions needed to help preserve this species in Illinois include protection of mature deciduous forest, especially along streams, long-term timber management and maintenance of favored tree species, including oaks, sycamores, elms and chestnuts.
Birds and people both depend upon forest resources. In order to conserve wood-warbler populations, land use decisions regarding forests must take into account the impacts of those uses on people, birds and the forest. Migratory birds are a shared international resource. Their population declines illustrate the global effects of human actions.