The Elfin-woods Warbler is a rare parulid that is endemic to a few upland forests of Puerto Rico. Its discovery in 1969 and description as a new species in 1972 was a surprising event. It was the first new species described from the West Indies since 1927, and the only in Puerto Rico in the 20th century (Kepler and Parkes 1972). Its historical range has been limited to middle and higher elevation humid habitats along the Cordillera Central, Sierra de Cayey and Sierra de Luquillo. Its existence at some of these sites has been questioned, and is thought to be restricted to Sierra de Luquillo (El Yunque Forest) in the east and Maricao Forest in the western end of the island. Recent counts in Sierra de Luquillo (El Yunque Forest) and Maricao Forest put the population at 1800 individuals. This warbler exhibits a preference for Podocarpus, Elfin and Palo Colorado forest types and occupies human-modified habitats at lower elevations and reduced abundances. The Elfin-woods Warbler feeds on arboreal arthropods captured at intermediate foliage heights, primarily by foliage-gleaning, sally-hovering from foliage and aerial sally-hovering. Elfins are monogamous. Breeding occurs from March to June, which coincides with the middle to latter part of the dry season. Cup nests are fitted snuggly inside dry and curled fallen leaves of Cecropia peltata that become entangled in vines and branches. Elfins also place cup nests inside tree cavities. Females lay 3-4 eggs. Adults feed the young and stay together in family groups from May to August. Family ties then disintegrate and young are chased and expelled from territories. The main threat to this species is the ongoing loss or modification of primary habitat, even in protected areas. These threats primarily are from the construction of infrastructure for the communication industry (i.e., television, microwave and cell phone towers) and for recreation facilities (picnic and camping areas), all of which has resulted in a reduction of preferred Elfin-woods Warbler habitat.