Sclater´s Tyrannulet

This is a small passerine bird belonging to the family Tyrannidae, measuring approximately 11 to 12 centimeters in length and weighing between 8 and 10 grams. It is distinguished by its olive-green upperparts and a contrasting dark grayish crown, which often features a semi-concealed white or pale yellow coronal patch. A prominent, well-defined whitish-yellow supercilium runs from the base of the bill to the back of the eye, bordered by a dark transocular stripe. Its wings are dark with two conspicuous yellowish-white wing bars and pale yellow margins on the flight feathers. The underparts are mostly pale yellow, fading to a grayer tone on the breast and a whitish throat. The bill is short, slender, and blackish, with a slightly decurved upper mandible tip. Both sexes are monomorphic, showing no significant differences in plumage or size.

Geographical distribution: It is patchily distributed along the eastern slopes of the Andes Mountains, ranging from southern Peru (specifically in the departments of Cuzco and Puno), through central and western Bolivia (La Paz, Cochabamba, Santa Cruz, Chuquisaca, and Tarija), to northwestern Argentina (provincias of Jujuy, Salta, Tucumán, and northern Catamarca). Two subspecies are currently recognized: the nominate Phyllomyias sclateri sclateri occupying the northern and central portions of the range, and Phyllomyias sclateri subares distributed in the southern regions of Bolivia and Argentina.

Habitat: This species primarily inhabits the canopy and sub-canopy of humid montane forests, especially within the Andean Yungas ecoregion. It operates across an elevational gradient typically spanning from 600 to 2,100 meters above sea level, though seasonal movements may occasionally push individuals slightly higher or lower. It favors mature primary forests, forest borders, well-developed secondary growth, and tree-dominated clearings, showing a strong avoidance of completely cleared agricultural lands.

Feeding: Its diet is omnivorous, consisting of a balanced mix of small insects and arthropods, alongside small berries and fruits from families such as Melastomataceae and Loranthaceae. Foraging occurs high in the trees, where the bird employs active techniques like leaf-gleaning, hover-gleaning, and brief sally-strikes to capture aerial prey or pluck ripe berries from the outer foliage.

Behavior: Highly active and restless, this tiny bird is usually encountered singly, in pairs, or as a regular member of mixed-species foraging flocks moving through the upper forest strata. It is relatively inconspicuous due to its preference for high canopy layers. However, its presence is readily detected by its characteristic vocalizations, which consist of a series of high-pitched, thin, descending whistles delivered in a rapid, rhythmic trill, primarily during the early morning hours.

Nesting: Breeding occurs during the warm, wet season from October to February, coinciding with peak food availability in the cloud forests. It constructs a tiny, delicate open cup nest woven from fine plant fibers, moss, and lichens, bound securely with spiderwebs to camouflage it against predators. The nest is placed on a high, horizontal branch, typically located more than 8 meters above the ground. The female lays a clutch of two creamy-white eggs, which she incubates, while both parents participate in feeding and protecting the nestlings.

Conservation status: It is currently assessed as Least Concern (LC) on the IUCN Red List of Threatened Species. While its population is estimated to be stable and it occupies a relatively wide geographical range, it faces localized threats from habitat loss and fragmentation driven by unsustainable timber extraction, agricultural expansion, livestock grazing in montane areas, and wildfires within the Yungas ecosystem.