Appearance

The genus Phylloscartes includes a diverse suite of small tyrant flycatchers, all of which have a relatively long tail and a slender bill; most species also have dusky mottling on the auriculars. As currently constituted, Phylloscartes includes two groups of behaviorally distinct groups of small tyrant flycatchers: tyrannulets ("true" Phylloscartes), which are more active foragers, with a horizontal (warbler-like) posture, and which frequently cock the tail; and the bristle-tyrants (formerly the genus Pogonotriccus), which have more upright posture, do not cock the tail, and are less active when foraging. Yellow-green Tyrannulet is a "true" Phylloscartes. It has olive upperparts, a white eyering, two prominent yellow wing bars, and yellow underparts. Unlike most other members of the genus, the auriculars do not have a strong pattern.

Shape and behavior are useful tools for identifying small tyrant flycatchers, including Yellow-green Tyrannulet. The horizontal posture, slender bill, and active movements of this species are more reminiscent of a gnatcatcher (Polioptila) or of a warbler (Parulidae) than they are of other sympatric species of small, yellow tyrant flycatchers. Yellow-crowned Tyrannulet (Tyrannulus elatus) has a more vertical posture, has a shorter tail that is not carried in a cocked position, and does not associate with mixed species flocks; it also has a distinctly gray cap and sides to the head, and a grayish throat. Yellow-olive (Tolmomyias sulphurescens) and Yellow-margined (Tolmomyias assimilis) flycatchers are larger, with much broader bills, more vertical posture and less active behavior, have indistinct pale edgings to the wing coverts but do not have strong wing bars, and have grayish crowns. Forest Elaenia (Myiopagis gaimardii) perhaps is the most similar small tyrant flycatcher, at least in terms of behavior, in that it associates with mixed species flocks and may assume a horizontal posture, but dusky sides to the crown, a grayish white throat and upper breast, and less distinct wing bars.

The following description is based on Ridgway (1907) and Wetmore (1972):

Adult: Sexes similar. Upperparts olive green, although the crown is very slightly grayer than the back. Rectrices grayish brown, outer webs edged with pale olive green. Wing coverts dusky, median and greater coverts broadly tipped with pale yellow, forming two distinct wing bars. Primaries dusky, edged with yellowish olive green; secondaries dusky, with broader, yellower edges. Inner margins of remiges edged with yellowish white. Lores grayish white, with a dark gray central spot. White eyering. Underparts, including the axillars and underwing coverts, pale yellow; chin faintly whiter, and upper breast faintly washed with olive.

Juvenile and immature: Undescribed.

Undescribed.

Iris: brown

Bill: black

Tarsi and toes: gray, black

Bare parts color data from Lawrence (1862) and Fitzpatrick (2004).

Total length: 10-11 cm (Wetmore 1972), 10.8 cm (Ridgway 1907)

Linear measurements (from Wetmore 1972):

male (n = 8):

wing length: mean 55.3 mm (range 52.5-57.5 mm)

tail length: mean 48.6 mm (range 47.0-51.8 mm)

bill length (culmen from base): mean 12.7 mm (range 11.2-13.5 mm)

tarsus length: mean 16.8 mm (range 16.2-17.5 mm)

female (n = 1):

wing length: 48.0 mm

tail length: 43.7 mm

bill length (culmen from base): 12.4 mm

tarsus length: 15.4 mm

Mass: 8.8 g (n = 1, female; Strauch 1977)