Black-capped Gnatcatcher
Polioptila nigriceps
The black-capped gnatcatcher is 10 to 12 cm (3.9 to 4.7 in) long and weighs 5 to 8 g (0.18 to 0.28 oz). Adults are blue-gray on the upperparts with white underparts. They have a long slender bill and a long black tail with mostly white outer feathers. Males in alternate (breeding) plumage have a glossy black cap. The female's gray upperparts have less of a bluish tint but are otherwise similar to the non-breeding male. This species is very similar to the black-tailed gnatcatcher (P. melanura). Voice: The black-capped gnatcatcher's song is complex and rambling . It also has a "mewing" call that is sometimes repeated .
ID text from Wikipedia, CC BY-SA 4.0
White-lored Gnatcatcher
Polioptila albiloris
The white-lored gnatcatcher is 11 to 12 cm (4.3 to 4.7 in) long and weighs 6 to 9 g (0.21 to 0.32 oz). The nominate male in breeding plumage has a black cap down to the eyes and including the nape. The rest of the upperparts are bluish gray. The tail is black with white outermost feathers. The throat is white becoming pale bluish gray on the breast and flanks. The female has a dark gray cap and the eponymous white lores and supercilium. The juvenile is similar to the female but has browner upperparts. The male P. a. vanrossemi has a larger black cap and longer wings and tail than the nominate. Voice: The white-lored gnatcatcher has two songs, a "simple" and a "complex" . It also has a variety of calls, for which see Xeno-canto below.
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-capped Gnatcatcher and White-lored Gnatcatcher.