Within the intermittently inundated grasslands of Bolivia, a diminutive, olive-green avian species has managed to elude scientific classification for an extended period. Following six decades of misdirection and inaccurate identification, ornithological experts have now affirmed that this bird represents not merely a localized variant of an existing member of the Hylophilus genus but an entirely distinct species. This significant finding contributes a novel endemic species to the already abundant avian tapestry of South America, emphasizing the vast expanse of the unknown, even within thoroughly researched avian families.
The Beni greenlet (Hylophilus moxensis) observed in its natural environment close to San Ramón, Beni, Bolivia, on January 10, 2024. Key distinguishing features include pale brownish-gray auriculars, contrasted by dark lores and a dark commissure base, with rufous coloration covering the entire crown and forehead, and dark irides. Photography courtesy of Tini Wijpkema.
This recently cataloged species is classified under the Hylophilus genus, a small assembly of passerine birds belonging to the Vireonidae family, which also encompasses vireos, greenlets, and shrike-babblers.
Designated scientifically as Hylophilus moxensis, and commonly referred to as the Beni greenlet, this avian inhabitant occupies the marshy shrublands characteristic of Bolivia’s Beni savannas, a sprawling and ecologically unique region also known as the Llanos de Moxos.
Despite initial observations by ornithologists dating back to 1960, the bird was long presumed to be an isolated population belonging to one of two similar Brazilian species: the rufous-crowned greenlet (Hylophilus poicilotis) or the gray-eyed greenlet (Hylophilus amaurocephalus).
“The morphological differences among many species within the Hylophilus greenlet genus are relatively understated; most exhibit shades of green, gray, yellow, and brown,” elucidated lead author Dr. Paul van Els, an ornithologist affiliated with the Museo Nacional de Historia Natural de la Paz, Sovon, and Anthus Bolivia, alongside his research collaborators in their published work.
“For certain species, the coloration of the iris can serve as one of the most effective identifiers to differentiate them from congeners with similar appearances.”
This period of taxonomic ambiguity was resolved when the research team undertook an in-depth genetic analysis, examining one mitochondrial gene and three nuclear genes across the relevant group.
The findings unequivocally demonstrated that the Beni population is not a variant of either previously recognized species. Instead, it stands as a sister taxon to the lineage comprising both Hylophilus poicilotis and Hylophilus amaurocephalus, representing a divergence event approximately 6.6 million years ago.
In contrast, the evolutionary split between the two species previously considered identical occurred much more recently, around 3.5 million years ago.
Dr. van Els and his co-authors further performed a meticulous examination of the facial plumage, ocular characteristics, and vocal repertoire of the species.
The Hylophilus moxensis can be definitively distinguished by its absence of dark markings on the ear coverts—a characteristic consistently observed in its closely related taxa—along with its uniform dark brown irises and unique song patterns, as detailed in the investigation.
In their analysis of vocalizations, the researchers identified that the species’ song features distinct ‘V-shaped notes,’ while its calls incorporate harmonic frequencies akin to those produced by female Hylophilus amaurocephalus. This specific combination of acoustic traits is not shared by either of the comparison species.
The identification of Hylophilus moxensis contributes to an expanding enumeration of endemic species found within the Beni savannas.
While the scientific team has indicated that they ‘do not currently perceive it to be under threat,’ they have also issued a cautionary note regarding the significant detrimental impact of extensive agricultural burning on the region’s biodiversity.
“The recognition of Hylophilus moxensis as an additional indigenous taxon in this area ought to galvanize conservation prioritization initiatives,” they articulated in their report.
“Unchecked burning for extensive agricultural operations presents a formidable challenge to the biodiversity of this region.”
“Although we are unable to provide a definitive population estimate for Hylophilus moxensis, our current assessment does not indicate an immediate threat, given the continued prevalence of suitable habitat throughout the region.”
“Nevertheless, the relatively limited number of recorded instances of this species might suggest factors beyond a mere scarcity of observers and could signify a genuinely localized distribution.”
The research team’s findings were disseminated online on January 1, 2026, within the peer-reviewed publication Avian Systematics, accessible via the paper.
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Paul van Els et al. 2026. A new species of greenlet from Bolivia in the Hylophilus poicilotis/amaurocephalus group (Vireonidae). Avian Systematics 3 (3): 17-37
