The people from Aves de Argentina were kind enough to identify this bird for me. It was seen in an area which contained Austral Thrushes, but the blue belly was (and is) a mystery to me. Later I found a book 'Birds of Neuquen (a state in Argentina) that contained a picture of this bird with a bluish belly, but I don't know if this is related to age, sex, mating colors, or a color variation.
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)