GNATCATCHER, the name given to birds of the American subfamily Polioptilinae of the warblers (Sylviidae). Some 15 species are recognized. Gnatcatchers are small, slender, greyish birds, and build beautiful lichen-covered nests. The best known species is the blue-grey gnatcatcher, Polioptila caerula, of the eastern United States, in which the male has a black fore head.