Masked Gnatcatcher
Polioptila dumicola
The masked gnatcatcher is 12 to 13 cm (4.7 to 5.1 in) long and weighs 5 to 7 g (0.18 to 0.25 oz), larger than most other gnatcatchers. The nominate male has a large black mask with a thin white line below it. Its forehead, crown, nape, and upperparts are blue-gray. Its tail is mostly black with white outer feathers. Its throat is pale gray fading to whitish on the belly. The nominate female does not have a mask but instead a black crescent down from the eye. Its upperparts are duller and not as bluish. P. d. Voice: The nominate masked gnatcatcher's loudsong is "variable, short, sweet and musical" . P. d. berlepschi's loudsong is quite different: . The species also has a variety of calls .
ID text from Wikipedia, CC BY-SA 4.0
Tropical Gnatcatcher
Polioptila plumbea
The adult tropical gnatcatcher is 10 to 12 cm (3.9–4.7 in) long, and weighs 6 to 8 g (0.21–0.28 oz). Its "jizz" is similar to that of other gnatcatchers; a small bird with a relatively long thin bill, a long frequently cocked tail, grey upperparts and whitish underparts. The central rectrices are black, while the outer are white (consequently, the tail appears primarily black from above, white from below), and there is a white patch in the wing (caused by broad white edging to the tertials).
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: Masked Gnatcatcher and Tropical Gnatcatcher.