Discovery and naming
The elfin woods warbler is one of many species in the genus Setophaga of the New World warbler family Parulidae. It was first observed in 1968 by Cameron and Angela Kepler while they were conducting observations on two Puerto Rican endemic birds, the Puerto Rican amazon and the Puerto Rican tody. On May 18, 1971, a specimen was captured in El Yunque National Forest, which at the time was believed to be its only habitat. A year later Kepler and Parkes described and named the species, making it the most recent warbler of the genus Setophaga discovered in the New World. Also, it is the first species described in the Caribbean since 1927 and the first Puerto Rican species described in the 20th century. The specific name, angelae, is a tribute to Angela Kepler. Elfin-woods warbler is an alternative spelling, and Reinita de Bosque Enano is the Spanish name. The species was initially placed in the genus Dendroica, but in 2011 the American Ornithologists' Union reorganized the classification of the family Parulidae and transferred species in Dendroica to Setophaga. This revised classification was adopted by the International Ornithologists' Union.
A phylogenetic analysis using mitochondrial DNA sequences from New World warblers has shown that, within the genus Setophaga, the elfin woods warbler is most closely related to the arrowhead warbler, a species which is endemic to Jamaica, and the plumbeous warbler, which is endemic to the islands of Dominica and Saint Lucia.