Greenish Elaenia
Myiopagis viridicata
The greenish elaenia is 13 to 14 cm (5.1 to 5.5 in) long and weighs 11 to 13 g (0.39 to 0.46 oz). The sexes have essentially the same plumage. Adults of the nominate subspecies M. v. viridicata have a greenish olive crown with a faint grayish tinge and a partially concealed bright yellow stripe along its middle. (Males have a larger yellow patch than females.) Adults have a dark loral streak, a whitish gray supercilium, a partial white eyering, and grizzled whitish ear coverts. Their upperparts are olive-green. Voice: The greenish elaenia's dawn songs and daytime calls seem to fall into four groups. The population in western Mexico sings a "high-pitched sweet tsewee-tseee....g'tseewe...tsewee-tseee...g'tseewe.... typically alternating a two-note and a one-note phrase". Its call is a "high-pitched two-note descending tsee-tseeu". The population in eastern Mexico sings a "moderately high pitched g'tswee-ee-ut...
ID text from Wikipedia, CC BY-SA 4.0

Pacific Elaenia
Myiopagis subplacens
The Pacific elaenia is about 14 cm (5.5 in) long; one male weighed 14.9 g (0.53 oz) and four females averaged 16.9 g (0.60 oz). The sexes have the same plumage. Adults have a gray-brown crown with a partially concealed bright yellow stripe along its middle. They have whitish lores, a partial white eyering, a grizzled whitish lower face with blackish ends on the ear coverts, and a wide whitish supercilium that continues down behind the black of the ear coverts. Their upperparts are brownish olive. Voice: The Pacific elaenia's dawn song is "an endlessly repeated 'chrrr, chrrr, che-wik, chrrr, chrrr, che-wik...' ". During the day it sings "a sharp, burry PJIT! djurrrree, the second note rising" and its calls include "a gruff pjrt and similar notes".
ID text from Wikipedia, CC BY-SA 4.0

Where you might meet both
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Play today's Daily BirdPhotos and recordings are credited to their authors under the licence shown. Full profiles: Greenish Elaenia and Pacific Elaenia.