About Polioptila plumbea (Gmelin, 1788)
The adult tropical gnatcatcher (scientific name Polioptila plumbea) measures 10 to 12 cm (3.9–4.7 in) in length and weighs 6 to 8 g (0.21–0.28 oz). It shares the general shape and habits (jizz) of other gnatcatchers: it is a small bird with a relatively long thin bill, a long tail that is frequently held cocked upward, grey upperparts, and whitish underparts. Its central tail feathers (rectrices) are black, while the outer tail feathers are white; as a result, the tail appears primarily black when viewed from above and white when viewed from below. There is also a white patch on the wing, created by broad white edging along the tertial feathers. Males of the nominate group have a contrasting black cap that extends just below the eye. Females of this group lack this black cap, and instead have a grey cap that matches the color of the bird's back. Some females have an irregular black patch behind the eye, and this marking is especially prominent in females of the subspecies atricapilla from north-eastern Brazil. Males of the bilineata group, commonly called white-browed gnatcatchers, resemble nominate group males, but the white of their underparts extends clearly above the eye. This produces a more restricted black cap, which is often connected by a fine black line running from the nape to the rear of the eye. Females of the bilineata group resemble males, but the black of the cap is replaced with grey, except for an irregular black patch behind the eye, which is absent in some females. Because white feathering extends above the eye in both sexes of this group, they have a distinctly white-browed appearance unlike the nominate group. This is the origin of the common name proposed for this group if it is recognized as a separate species. The tropical gnatcatcher produces a thin buzzy gezzz call and a trilled swee see see si si si su su song. The exact structure and tone of its vocalizations varies greatly across the species' range. The nominate group occurs from north-eastern Brazil's Caatinga region, west through the Amazon Basin, and north to the Guianas, northern Venezuela, and northern Colombia, including the Magdalena and Cauca river valleys. The bilineata group is found from north-western Peru, through the Chocó region and Central America, to southern Mexico. Both groups occupy a wide range of wooded habitats, from arid woodland and scrub such as Brazil's Caatinga to humid forest such as the various humid forest types of the Amazon. Most populations are mainly found in lowlands below 1,000 m (3,300 ft) in altitude, but the taxon maior, which is restricted to dry woodland and scrub, occurs at altitudes between 200 and 2,700 m (660–8,860 ft). In dense humid forest, this species is typically found in the canopy, but it is commonly seen at lower elevations in more open habitats. The tropical gnatcatcher gleans spiders, spider eggs, beetles, caterpillars, and other insects from outer twigs and foliage. It moves around alone or in pairs, and often joins mixed-species feeding flocks that include tanagers, New World warblers, and honeycreepers. It builds a small cup-shaped nest similar to that of a hummingbird, made from vegetable fibers, placed 2.0 to 8.5 m (6.5 to 28 ft) high on a tree branch. The female lays two or three brown-spotted white eggs in May and June. This species is curious and unafraid of humans, but this behavior makes it more vulnerable to predators. Even small omnivorous mammals like the common marmoset (Callithrix jacchus) will readily plunder tropical gnatcatcher nests in the undergrowth. This is more common during the dry season when fruits are scarce, despite the parent birds' attempts to defend their offspring.
