Introduction
The Slender-tailed Woodstar (Microstilbon burmeisteri) is a tiny hummingbird and the only species in the genus Microstilbon. It is restricted to the eastern Andes of Bolivia and north-western Argentina, where it inhabits wooded slopes, shrublands and forested ravines. Despite its small size and inconspicuous habits, it is recognisable for the male’s glittering reddish-purple throat and unusually narrow, forked tail.
Physical Description
The Slender-tailed Woodstar measures approximately 7–8 cm in length. Both sexes have bronzy-green upperparts, a short, straight black bill and a narrow pale stripe behind the eye.
The male has a glittering reddish-purple gorget that extends into elongated side tufts, sometimes described as moustache-like feathers. The remainder of the throat is white, while the underparts are pale grey with green mottling on the flanks. The undertail coverts are pale cinnamon, and the long, narrow tail is deeply forked and predominantly black.
The female lacks the colourful gorget. Her underparts are more cinnamon-buff, and her tail is shorter, with green central feathers, cinnamon outer feathers and a dark band near the tip. Juvenile plumage is less well documented, but young birds are expected to resemble females before acquiring adult male characters.
Range and Habitat
The species occurs from the departments of Cochabamba and Santa Cruz in Bolivia southwards into the Argentine provinces of Jujuy, Salta and Tucumán. An older record has been reported from the La Paz region of Bolivia, but the species is not currently regarded as widespread there. It is not known from Colombia, Ecuador, Peru or Venezuela, as sometimes stated in older or poorly sourced accounts.
Slender-tailed Woodstars use a variety of Andean habitats, including deciduous woodland, montane forest margins, thorn scrub, shrubby slopes and densely vegetated ravines. They have been recorded mainly between about 1,100 and 3,000 metres above sea level, although the exact limits vary between localities. The species appears to be resident in parts of its range, but seasonal movements along elevational gradients are suspected and remain poorly understood.
Where to See This Species
Parque Nacional Calilegua, Jujuy, Argentina
The species is documented from Parque Nacional Calilegua, a protected area containing Yungas forest, montane woodland and transitional habitats suitable for this hummingbird. Look along forest edges, wooded tracks and flowering vegetation, particularly where dense vegetation meets more open scrub.
Nearby accommodation: The practical gateway is Libertador General San Martín, where visitors can find hotels, guest houses and self-catering accommodation. Camping may also be available near the park subject to current local arrangements.
Parque Nacional El Rey, Salta, Argentina
Parque Nacional El Rey supports documented populations in montane forest, transitional woodland and areas containing pino del cerro habitat. The species may be difficult to locate because of its size and fast flight, so quiet observation around flowering shrubs and forest clearings is advisable.
Nearby accommodation: The nearest practical base is the city of Salta, with additional rural accommodation available around the approach settlements and access routes to the park. Camping within or near the park should be checked with the park authority before travel.
Parque Nacional Los Cardones, Salta, Argentina
Slender-tailed Woodstar is recorded from Parque Nacional Los Cardones, where suitable habitat includes montane woodland, scrub and vegetated ravines at middle and higher elevations. It is most likely to be encountered where woodland and shrub communities provide flowering plants rather than in the park’s most exposed high-altitude landscapes.
Nearby accommodation: Cachi and Payogasta are the main practical gateway settlements, offering guest houses, small hotels, cabins and self-catering accommodation.
Behaviour and Diet
The Slender-tailed Woodstar is an active hummingbird that often forages high in vegetation. Its rapid wingbeats can make it resemble a large bee at a distance. It feeds primarily on nectar, including nectar from epiphyte flowers, and probably visits a range of other flowering plants. Small insects and other tiny arthropods are also taken, providing an important source of protein.
The species is reportedly subordinate to several larger hummingbirds and may be displaced from feeding sites. It can produce short, subdued chip notes while feeding or hovering. Detailed information about territoriality, courtship and social behaviour remains limited, and the elaborate aerial displays described for some other hummingbirds have not been adequately documented for this species.
Breeding records are scarce. The known breeding season includes December and January. One recorded nest was a small cup made from soft plant fibres and covered with lichens, attached to a branch with spider silk. The clutch is reported to contain two white eggs, but incubation and fledging periods are not well established.
Identification Tips
- Look for a very small hummingbird with a short black bill, bronzy-green upperparts and a pale stripe behind the eye.
- The male’s reddish-purple throat is the most distinctive feature, with elongated side feathers that can give the appearance of small moustache tufts.
- The narrow, deeply forked tail is particularly useful when the bird is seen from behind or in flight.
- Females lack the purple gorget and appear more cinnamon below, with a shorter tail and a dark band near the tail tip.
- Do not confuse the species with other small woodstars solely on size. The combination of the male’s side-extending gorget, pale eye stripe and slender forked tail is more reliable than size alone.
- Because it often moves quickly through dense vegetation, brief views may show only a small green hummingbird with a narrow tail. Repeated observations of the tail shape and the location of any purple throat feathers can help confirm identification.
Conservation Status
The Slender-tailed Woodstar is currently assessed as Least Concern on the IUCN Red List. Its global population size has not been quantified, but available assessments consider the population broadly stable. The species is regarded as locally common in parts of its range, although it is easily overlooked because of its small size and secretive behaviour.
Habitat loss and degradation remain potential local threats, particularly where montane woodland and Yungas habitats are cleared for agriculture, affected by intensive grazing or fragmented by development. Protected areas in Argentina provide important safeguards, but continued protection of native woodland, shrubland and flowering plant communities is needed. Further field research would improve understanding of population size, seasonal movements, breeding success and habitat requirements.
Frequently Asked Questions
Where can the Slender-tailed Woodstar be found?
It occurs in the eastern Andes of Bolivia and north-western Argentina, particularly in parts of Jujuy, Salta and Tucumán. It is associated with montane woodland, forest edges, scrub and vegetated ravines.
How large is the Slender-tailed Woodstar?
It is approximately 7–8 cm long. Its small size, rapid wingbeats and narrow forked tail make it easy to overlook among dense vegetation.
How can males and females be distinguished?
Males have a glittering reddish-purple gorget with elongated side feathers. Females lack this throat patch and are generally cinnamon-buff below, with a shorter tail and a dark band near the tip.
What does the Slender-tailed Woodstar eat?
It feeds mainly on nectar from flowering plants and also takes small insects and other tiny arthropods. It has been recorded foraging on flowers in epiphytes and other vegetation.
Is the Slender-tailed Woodstar migratory?
Its movements are not fully understood. It is probably resident in at least part of its range, but seasonal movements up and down mountain slopes are suspected.
Is the Slender-tailed Woodstar endangered?
No. It is currently classified as Least Concern, although habitat loss and degradation can affect local populations and remain important conservation concerns.
References
- BirdLife International
- Cornell Lab of Ornithology, Birds of the World
- Sistema de Información de Biodiversidad, Administración de Parques Nacionales, Argentina
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