Black-whiskered Vireo
Vireo altiloquus
The black-whiskered vireo is 15 to 16.5 cm (5.9 to 6.5 in) long. The nominate subspecies V. a. altiloquus weighs an average of 18 g (0.63 oz). The sexes have the same plumage. Adults of the nominate subspecies have a bluish gray forehead and crown with a buff-olive tinge, a buffy whitish supercilium from the bill past the eye, a wide dusky stripe through the eye, and the eponymous black "whisker" on the sides of the throat. Their nape and upperparts are olive-green. Voice: Migratory black-whiskered vireos sing during the breeding season and are silent on the wintering grounds. Nonmigratory populations sing year-round but more frequently during the breeding season. In the West Indies the black-whiskered vireo sings monotonously throughout the day, "hort, melodious 2- to 3-syllable phrases, each different, separated by pauses".
ID text from Wikipedia, CC BY-SA 4.0

Yellow-green Vireo
Vireo flavoviridis
The adult yellow-green vireo is 14–14.7 cm in length and weighs 18.5 g. It has olive-green upperparts and a dusky-edged gray crown. There is a dark line from the bill to the red-brown eyes, and a white supercilium. The underparts are white with yellow breast sides and flanks. Young birds are duller with brown eyes, a brown tint to the back, and less yellow on the underparts. The adult yellow-green vireo differs from the red-eyed vireo in its much yellower underparts, lack of a black border to the duller gray crown, yellower upperparts and different eye color.
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: Black-whiskered Vireo and Yellow-green Vireo.