Mountain Elaenia
Elaenia frantzii
The mountain elaenia is 14 to 15 cm (5.5 to 5.9 in) long and weighs about 14 to 24 g (0.49 to 0.85 oz). It is a small to medium size elaenia with a rounded crest. The sexes have the same plumage. Adults of the nominate subspecies have a brownish olive head with lighter cheeks, a thin very pale olive eyering, and a mostly hidden white patch on the crown. Their upperparts are grayish olive. Their wings are mostly dusky with pale olive or olive-yellow edges on the flight feathers. The tips of their wing coverts are yellowish olive and show as two bars on the closed wing. Voice: The northern and southern pairs of subspecies of the mountain elaenia have somewhat different vocalizations, which is part of the evidence hinting that they are separate species. The dawn song of the northern pair is "an endless repetition of an overslurred whistle followed by a rising burry part and a higher-pitched second overslurred note, all three notes interconnected without a clear pause".
ID text from Wikipedia, CC BY-SA 4.0
Yellow-bellied Elaenia
Elaenia flavogaster
The yellow-bellied elaenia is 15 to 17 cm (5.9 to 6.7 in) long and weighs 21 to 29 g (0.74 to 1.0 oz). It is medium-sized, has a small head, and a bushy crest. The sexes have the same plumage. Adults of the nominate subspecies E. f. flavogaster have a brownish olive crown with a partially concealed white stripe in the middle of the crest. They have whitish lores and a faint whitish eyering on an otherwise pale brownish olive face. Their upperparts are brownish olive. Voice: The yellow-bellied elaenia's dawn song has been rendered as "trr-dyeéuw, trr-trreeenh-weeeuw" or "we-do, we-do". Other renderings are "a series of burry teeotree or tee-tree notes interspersed with an occasional wurrTREE" and "a leisurely and buzzy spud-deeer, spud-deer-dzz, spud-deer... over and over".
ID text from Wikipedia, CC BY-SA 4.0
Where you might meet both
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Play BeakBrainPhotos and recordings are credited to their authors under the licence shown. Full profiles: Mountain Elaenia and Yellow-bellied Elaenia.