Straneck's Tyrannulet
Serpophaga griseicapilla
Straneck's tyrannulet is 9 to 11 cm (3.5 to 4.3 in) long and weighs 4.4 to 7.5 g (0.16 to 0.26 oz). The sexes have the same plumage. Adults have a gray crown and nape with a hidden bit of white on some black central crown feathers. They have a thin white supercilium, a white crescent under the eye, and a dark gray line through the eye on an otherwise grayish white face. Their upperparts are olivaceous brown. Their wings are dusky brownish gray with lighter edges to most of the flight feathers and two cinnamon to whitish wing bars. Voice: The main vocalization of Straneck's tyrannulet is "a high-pitched but not far-carrying 'twie tirrrrrrrrrrrrrrrrrr', occasionally with the first or second portions emitted alone". Its alarm call is "a more powerful, high-pitched 'tie...tie...tie...tie...' " of three to five notes.
ID text from Wikipedia, CC BY-SA 4.0
White-crested Tyrannulet
Serpophaga subcristata
The white-crested tyrannulet is 11 cm (4.3 in) long and weighs 6 to 8.6 g (0.21 to 0.30 oz). The sexes have the same plumage. Adults of the nominate subspecies have a gray crown and nape with white central crown feathers that are usually hidden but sometimes raised as a crest. They have a thin white supercilium, a white partial eye-ring, and a dusky line through the eye on an otherwise grayish white face. Their upperparts are grayish olive. Their wings are dusky to blackish with pale buffy edges to most of the flight feathers and two whitish to pale buffy wing bars. Voice: The white-crested tyrannulet's song differs somewhat among the subspecies. That of the nominate is described as an "almost mechanical-sounding trill with few introductory rising notes, psee psee psee-e-e-e-e-e-e". It also makes a "slower, bubbly tweetweetweetweetweetwee, and occasionally other, more jumbled chatters". The song of S. s.
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: Straneck's Tyrannulet and White-crested Tyrannulet.