This species was known about for many years, but due to confusion, mislabeled specimens, and some naming problems, it was not "officially" accepted as a real species until quite recently. See SACC proposal 419 for more info. This individual is fairly washed out, normally they look a little more yellow on the underparts and wingbars, but it does have the feature of showing very little white in the crest. It was also calling. Thanks to Mark Pearman for confirming the ID.
Straneck, 2007: NOTE(NEW: Straneck's Tyrannulet is a newly described species with distinctive vocalizations and confused taxonomic history: Straneck, 2007 (SACC 419).)
German
Grauscheitel-Kleintyrann
French
Tyranneau de Straneck
Chinese
灰冠姬霸鹟
Japanese
ハイガシラカトリタイランチョウ
Russian
Серошапочная серпофага
Spanish
Piojito de Straneck
Portuguese
alegrinho-trinador
Bird Data App text
Very similar to the [White-crested Tyrannulet]. {Austral migrant} from Argentina.
Africa (entire continent rather than south of Sahara)
AN
Antarctica
AO
Atlantic Ocean
AU
Australasia (Wallacea (Indonesian islands east of Wallace's line), New Guinea and its islands, Australia, New Zealand and its subantarctic islands, the Solomons, New Caledonia, and Vanuatu)
PAL
Eurasia (Europe, Asia from the Middle East through central Asia north of the Himalayas, Siberia and northern China to Japan)
IO
Indian Ocean
LA
Latin America (Middle and South America)
MA
Middle America (Mexico through Panama)
NA
North America (includes the Caribbean)
NO
Northern oceans
OR
Oriental Region (South Asia from Pakistan to Taiwan, plus Southeast Asia, the Philippines, and Greater Sundas)