A mockingjay is a bird that was created through mating of jabberjays and female mockingbirds. When Katniss Everdeen wore a pin with this bird on it in the 74th Hunger Games, the Capitol was angry as it represented a slight against them due to the mistake that caused their existence. This is why Katniss became "The Mockingjay" and led the rebels in Mockingjay.
A hybrid of mockingbirds and jabberjays, the birds were created by mistake. The jabberjays - all male - were initially created to eavesdrop on the rebels in the Dark Days, memorizing entire conversations and repeating them back at the Capitol. Once they were discovered, the rebels fed endless lies to the birds, and sent them back loaded with false information. After the lies were discovered, the Capitol closed the laboratories and the jabberjays were released into the wild, in the hope that they would die off. They did, eventually, but not before they passed on their genetic code to female mockingbirds. This was unforeseen, because no one expected them to be able to reproduce with other bird species. The offspring were called mockingjays, and, while they had lost the ability to memorize words, they could mimic any sounds from a child's high-pitched warble to a man's deep tones, and even songs with multiple verses, if you had the patience to sing to them. Katniss' father was one of those people, and Peeta claims that when he first heard Katniss sing, all of the birds, including mockingjays, fell silent, the sign that they liked the song or notes that were sung. The mockingjay is so important to Katniss because her father had a special bond with them, and so did Rue.
Like their jabberjay fathers, mockingjays are excellent mimics, and have the ability to memorize and repeat both bird and human songs. They can perfectly copy down to the last note, any human tune. They pick up tunes quickly, and often spread them to other mockingjays. However, they only sing the songs of those whose voices they enjoy. Katniss, her father, and Rue are singers that caused the mockingjays to sing. Katniss also notes that they can sing both high and very low pitches.
In District 11, the first worker to see the quitting signal whistles four notes. The mockingjays will then begin to spread the notes, passing the message along to all the workers. The simple melody is ingeniously designed to harmonize with itself as the different voices singing it overlap. Rue practiced this, and may have even begun it.
The birds are found in District 11, in the orchards. They are also found in various other districts, such as District 13, as well as outside District 12's borders. Rue is said in the book to tell Katniss that they are a way of communicating through the orchards in District 11.
Mockingjays are described as being as tough as rocks, being able to thrive in almost any environment.
In the 74th Hunger Games, there were mockingjays in the arena. Rue and Katniss used them to send a signal to each other saying they were alright.
Symbol of rebellion
Mockingjays have a certain level of symbolism in Panem. It stands for a very exciting Hunger Games and a symbol of rebellion. Wearing a mockingjay pin is a sign of rebellion to the Capitol — whether or not the wearer means any harm. Not only did it fail in killing off their own creation, but it turned into something even better than before. The jabberjay mating was a symbol of justice, adding to the rebellion.
The mockingjay pin that Madge Undersee gave Katniss Everdeen was no exception. It originally belonged to Madge's aunt, Maysilee Donner, who died in the 50th Hunger Games. The pin, chosen for Katniss' one token of home, the woods outside of District 12, she found two women from District 8, Bonnie and Twill who were rebels on a journey to District 13 because they saw a mockingjay in the commercials. Plutarch Heavensbee had a watch that when glistened, showed a hint of a mockingjay. He showed this to Katniss not only to hint that he was on her side, but also to hint the arena in the 75th Hunger Games.
In The Hunger Games: Mockingjay - Part 1, President Snow banned any references to the Mockingjay symbol due to the rebellion. He threatened offenders with death. During his speech, Snow's granddaughter, who is in the same room as Snow, can be seen unbraiding her hair, as it was inspired by Katniss, the "mockingjay" of the rebellion.