What is the symbolism of the pretty bird on the front cover of Mocking Jay?

The Mockingjay, in the novel
Mockingjay (of The Hunger Games trilogy)
symbolizes many different things.

Katniss' first real brush
with the Mockingjay happens in novel one.  She receives a pin with the mockingjay on it
from the mayor’s daughter, Madge Undersee. Also during her first appearance in the
Hunger Games, Katniss befriends a young tribute from District 11, Rue. Rue reminds
Kantiss of her sister (Prim) and they create a way to talk back and forth during the
games using bird calls.

As the novels move on, Katniss
comes to know that the mockingjay came about after a bad decision of the Capitol's. (The
Capitol created the jabberjay, they escaped from the Capitol, and mated with
mockingbirds.) Katniss is, therefore, symbolic of the mockingjay herself.  She is,
ultimately, a creation of the Capitol after her trip to the Hunger Games. She was
changed and became something completely different than what she was prior to the
games.

For the final novel, the mockingjay represents the
rebellion against the Capitol.  Katniss, being the perfect symbol to represent the
rebellion given her dislike for the Capitol, becomes the
Mockingjay.