A chicken was a bird that was raised primarily for food, usually eaten by Humans or Urgals.
In Eldest, a man named Gamble ate thirteen chickens in only four days. He was brought before Nasuada and forced to pay a copper for each. He was given a higher paying job to ensure he did not steal more food.
In Brisingr, Eragon absorbed energy from dying chickens to put into the Belt of Beloth the Wise in order to conserve energy.