This is my and Argentine bird expert JC Grasso's best guess at identification. In this region with white eyering, Yellow-bellied Elaenia, Lage Elaenia and White-crested Elaenia are also possible.
A slightly larger and a bit slow-moving elaenia. Note small head in relation to body, especially of the 'sordida' subspecies, sometimes split as the Small-headed Elaenia.
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)