White-crested Tyrannulet

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. Their tail is dusky with whitish edges on the outer feathers. Their chin and throat are white, their breast gray, and their belly pale yellow. Juveniles have buff wing bars and little or no white on the crown. Subspecies S. s. straminea is very similar to the nominate but with a brighter yellow belly. S. s. munda is larger than the nominate. Its upperparts are gray with a faint olive tinge, its wing bars are white, and its belly is white with sometimes a faint creamy yellow tinge. Both sexes of all subspecies have a dark brown iris, a black bill, and gray legs and feet.

The nominate subspecies of the white-crested tyrannulet is found from Bolivia's La Paz Department east to southern Mato Grosso in Brazil and south through Paraguay to northeastern Chubut Province in Argentina. Subspecies S. s. straminea is found in Uruguay and eastern Brazil. S. s. munda is found from western Bolivia and Argentina west of straminea and migrates east to north-central Bolivia, southeastern Brazil, and Uruguay. In all seasons the species inhabits a variety of landscapes including the edges of tropical forest, deciduous woodlands, gallery forest, thorn forest, dryish scrublands, cerrado, plantations, gardens, and agricultural areas. In the non-breeding season it tends to prefer the more open landscapes like scrublands and agricultural areas. In elevation it ranges from sea level to 2,800 m (9,200 ft) in Brazil, and up to 700 m (2,300 ft) in Argentina and 2,900 m (9,500 ft) in Bolivia.

The white-crested tyrannulet's diet has not been detailed but is believed to be entirely small insects. It is an active forager, flitting around in scrubby vegetation. It takes prey by gleaning while perched and while briefly hovering after a short flight; it also takes some prey in mid-air.