According to Pixar's website, the first four birds actually have names and individual characteristics. From left to right they are: Chipper, Bully, Snob and Neurotic. Look closely and you'll see them act out their personalities.
This represented two major technological challenges for the company - the animation of feathers and a continuously moving background, in this case, a rolling wheat field.
After experiencing great success with their animated shorts, John Lasseter opened up the department's facilities to anyone in the company who had a good idea for the next short. Ralph Eggleston, who had joined the company in 1993 to oversee art design on Toy Story (1995), had this idea from his CalArts days that he hadn't been able to complete.