A Ballad of Songbirds and Snakes is the sequel of Hunger Games revealed the real reason why the Mockingjay was so powerful symbol of resistance. Katniss Everdeen earned the outrage of the Capitol unintentionally after wearing a pin portraying a Mockingjay. Since then, it persisted become the symbol of the rebellion against President Snow. 

From the previous books and movies, it is not unknown that Mockingjays were a fortuitous mutation created by the Capitol during their war with the Districts. The Capitol genetically engineered a new species of birds called jabber jays, which aspired to be a biological spying characteristic. The Jabber jays would spread through the forests recording surrounding chatter they heard- confidently included rebel secrets. Then they recalled the recordings at night to the Capitol insight into the rebellion’s plans. 

Regrettably, the Capitol couldn’t judge the Jabber jays mating with female mockingbirds and creating a completely new race- the Mockingjay. The Mockingjay became a metaphor of a world beyond their control leading to a challenge to their authority.