The term 'Big South American Bird' can be summarized by the crossword solution RHEA, which is a four-letter word. RHEA is a genus of large flightless birds native to South America. They are the largest living bird in the Americas and the only extant member of the Rheidae family. Rheas are omnivorous and feed on a variety of plants, insects, and small animals. They are also known for their ability to run quickly and can reach speeds of up to 40 mph.