Muttations are mutants created by the Capitol, often for the purpose of given people living in the Districts grief. During the Dark Days the Capitol created the Jabberjay, a bird that would repeat everything it hear. They were used for spying on the rebels. This plan however backfired when the rebels realized what was happening and started saying rubbish around the birds, leading the Capitol on many false trails. They are used in the Clock Arena during the 3rd Quarter Quell as one of the twelve sections of the clock. The Jabberjays imitate the sounds of the tribute's loved ones being tortured.
Tracker Jackers are another muttation that features heavily in the series. In The Hunger Games Katniss drops a tracker jacker nest on the Career group, including Peeta. Tracker jackers are like wasps, though their stings are poisonous and if the poison doesn't kill the target it causes them to hallucinate. They are also bigger than regular wasps and attack anything that disturbs them once they zone in on it (i.e. they 'track' people, thus their name).