Only a few weeks ago they were crossing the Gulf of Mexico in mixed-species flocks of several thousand or more, each bird measuring just four to six inches long and weighing perhaps two ounces. Some, like the blackpoll warbler, spend the winter as far south as Peru, and in a matter of weeks reach their summer nesting grounds in Alaska, Canada, and the northeastern United States. Warblers are a strictly New World family of birds, with some 100 species in the Americas, about 40 of which are known to occur in North America. Primarily birds of forest and woodlands, warblers reach their greatest abundance and diversity across the eastern half of the United States, which supports more lush and diverse forest communities.
Warblers also have the distinction of being fairly sensitive to changes in their habitats.
Some species nest on the ground, or close to it in tree stumps and low shrubs, which makes them vulnerable to predation. Many insist upon the solitude and security of large, unbroken tracts of forest. When these areas become fragmented through conversion to human uses, these species are pushed nearer to the forest edge, where their nests are at risk for sabotage by the brown-headed cowbird. A much larger, parasitic bird, the female cowbird lays a single egg of her own in the nests of warblers and other songbirds. In some cases she will smash and devour the host birds' eggs.