The condition of an animals liver was said to hold answers. Eagles represented imperial power, storms indicated an angry god. Wolves were symbolic of the city of Rome. The actions of these things were often interpreted as warnings of future events. There must have been plenty of others as well.
Oh, and concerning chickens, one roman admiral didn't like the augury the chickens gave him, and had the lot thrown overboard. He said something along the lines of "What do they know, they're only chickens". He then lost the battle.