The fan-tailed warbler (Basileuterus lachrymosus) is a species of bird in the family Parulidae, the New World warblers. It is found from Mexico to Nicaragua. It has also been recorded as a vagrant in the U. S. states of Arizona, New Mexico, and Texas. Its "habit of constant tail-fanning and tail-swinging [is] distinctive".

Taxonomy and systematics

The fan-tailed warbler has a complicated taxonomic history that as of July 2026 has not been resolved. Its formally description in 1850 with the binomial Sylvia lachrymosus is credited to Charles Lucien Bonaparte. However, Bonaparte appended "(Basileuterus? lachrymosus, Cabanis)" to the binomial. Cabanis had not published the name and so is not credited with the description despite the species' later assignment to his genus.Curson, J. and G. M. Kirwan (2025). Fan-tailed Warbler (Euthlypis lachrymosa), version 1.1. In Birds of the World (J. del Hoyo, A. Elliott, J. Sargatal, D. A. Christie, and E. de Juana, Editors). Cornell Lab of Ornithology, Ithaca, NY, USA. https://doi.org/10.2173/bow.fatwar.01.1 retrieved July 21, 2026 In 1850 Cabanis had also erected genus Euthlypis, and in 1902 Robert Ridgway assigned the fan-tailed warbler to it, retroactively declaring it the type species of the genus.

A molecular phylogenetic study published in 2010 found that the fan-tailed warbler was basal to the genus Basileuterus. Based on the study, in 2011 taxonomic systems began reassigning it to that genus.R. Terry Chesser, Richard C. Banks, F. Keith Barker, Carla Cicero, Jon L. Dunn, Andrew W. Kratter, Irby J. Lovette, Pamela C. Rasmussen, J. V. Remsen, Jr., James D. Rising, Douglas F. Stotz, and Kevin Winker. "Fifty-second supplement to the American Ornithologists' Union Check-list of North American Birds". The Auk 2011, vol. 128:600–613 retrieved October 22, 2025 As of July 2026 the IOC, BirdLife International's Handbook of the Birds of the World (HBW), and AviList retain it in Basileuterus.HBW and BirdLife International (2025). Handbook of the Birds of the World and BirdLife International digital checklist of the birds of the world. Version 10. Available at: https://datazone.birdlife.org/about-our-science/taxonomy#birdlife-s-taxonomic-checklist retrieved October 12, 2025 However, based on a 2024 molecular phylogenetic study, in 2025 the American Ornithological Society and the Clements taxonomy returned it to genus Euthlypis.R. Terry Chesser, Shawn M. Billerman, Kevin J. Burns, Carla Cicero, Jon L. Dunn, Blanca E. Hernández-Baños, Rosa Alicia Jiménez, Oscar Johnson, Nicholas A. Mason, and Pamela C. Rasmussen. "Sixty-sixth supplement to the American Ornithological Society's Check-list of North American Birds". Ornithology 2025, vol. 142:1-19 retrieved September 5, 2025Clements, J. F., P. C. Rasmussen, T. S. Schulenberg, M. J. Iliff, J. A. Gerbracht, D. Lepage, A. Spencer, S. M. Billerman, B. L. Sullivan, M. Smith, and C. L. Wood. 2025. The eBird/Clements checklist of Birds of the World: v2025. Downloaded from https://www.birds.cornell.edu/clementschecklist/download/ retrieved November 3, 2025 In November 2025 AviList received a proposal to decide which genus to use.

All of the taxonomic systems agree that the fan-tailed warbler is monotypic.

Description

The fan-tailed warbler is about long and weighs . The sexes have the same plumage. Adults have a black forecrown, a narrow yellow crown patch, a white spot on the lores, and thin white crescents above and below the eye on an otherwise dark gray face. Their upperparts and wings are dark gray with a faint olive wash on the mantle. Their tail is long, graduated, and dark gray with white feather tips. Their throat, breast, and belly are rich yellow, their flanks are washed with tawny-yellow, and their undertail coverts are white. Birds in the southern part of their range have more black on their face, are darker gray above, and have a richer tawny wash than more northerly birds, though the differences are clinal. They have a dark iris, a black bill, and dusky pinkish legs and feet. Juveniles have an ash-gray head and upperparts. Their wings' greater and median coverts have buff tips that show as two thin wing bars. Their throat, breast, and flanks are brownish gray and their belly and undertail coverts pale buffy olive. They have a paler bill and legs than adults.

Distribution and habitat

The fan-tailed warbler is found "on the Pacific slope of Mexico from southern Sonora south to Guatemala, El Salvador, and Honduras (locally also in interior valleys) to central Nicaragua and on the Gulf slope of Mexico from eastern San Luis Potosí and southern Tamaulipas to Veracruz and north-central Oaxaca." South of Mexico is it found intermittently. It has also been recorded as a vagrant in the U. S. states of Arizona, New Mexico, and Texas.

The fan-tailed warbler inhabits semi-deciduous and evergreen forest with an intact understory in the foothills and lower mountain slopes. It favors "rugged, rocky areas such as ravines and forest-covered lava flows". Overall it ranges in elevation from . In El Salvador, Guatemala, and Honduras it ranges from .

Behavior Movement

The fan-tailed warbler is generally considered to be a year-round resident, though some part of the populations in Sonora and Chihuahua may withdraw south in winter.

Feeding

The fan-tailed warbler feeds on insects and other invertebrates. It takes most prey on the ground and in low vegetation by gleaning from leaves and rocks. It regularly attends army ant swarms to capture prey fleeing from the ants; up to 12 individuals have been seen doing so. It has also been observed several times following nine-banded armadillos (Dasypus novemcinctus), apparently for the same reason. In the non-breeding season individual birds and pairs may join mixed-species feeding flocks.

Breeding

The fan-tailed warbler's breeding season has not been defined. Its nest is an open cup made from dry leaves, grass, and pine needles. It is usually well-hidden on the ground, and often under leaf litter. Nothing else is known about the species' breeding biology.

Vocalization

One description of the fan-tailed warbler's song is "a series of clear, downslurred whistles, rising sharply on penultimate note...as wee wee wee wee wee wee-cher". It also sings "a more variable series on more even pitch and with slow terminal flourish". Its calls are "a thin, high-pitched tseeng or schree, also a shorter si". Another description and transcription of the song is "a pleasant sharply whistled see'sah'see'sah'see'seew'seew" whose last note drops in pitch. This author describes its call as "a thin, high-pitched seeeuh".

Status

The IUCN has assessed the fan-tailed warbler as being of Least Concern. It has a very large range; its estimated population of at least 50,000 mature individuals is believed to be decreasing. No immediate threats have been identified. It is considered generally fairly common but local in some areas.

References