What is the connection between the whole story of To Kill a Mockingbird and birds?

Harper Lee uses bird symbolism throughout To
Kill a Mockingbird, establishing the mockingbird as a symbol of innocence
after Jem and Scout receive air rifles for Christmas. Atticus tells Jem to shoot all the
blue jays he wants,

readability="6">

"... but remember, it's a sin to kill a
mockingbird."

Miss Maudie
elaborates Atticus' warning when she explains to Scout
that

readability="9">

"Mockingbirds don't do one thing but make music
for us to enjoy. They don't eat up people's gardens, don't nest in corncribs, they don't
do one thing but sing their hearts out for us. That's why it's a sin to kill a
mockingbird."

When death is
eminent for Tim Johnson, the mad dog who Atticus is forced to kill, the innocent
"mockingbirds were silent," as if already in mourning for the loss of a fellow animal.
As the jury prepares to give their verdict, Scout senses that
the

...
atmosphere in the courtroom was exactly the same as a cold February morning, when the
mockingbirds were still...

No
birds sing for Tom, the innocent victim of malicious charges by the Ewells. After Tom's
death, B. B. Underwood's editorial likened Tom's killing
to

... the
senseless slaughter of songbirds by hunters and
children.

Even Scout
recognizes the validity of Atticus' warning after Sheriff Tate decides to call Bob
Ewell's death self-inflicted. Charging Boo Radley--another of the novel's innocent human
mockingbirds--with Bob's death would have been

"... like
shootin' a mockingbird." 

Lee established her bird
symbolism with the main characters, the Finch family. The finch, like a mockingbird, is
a songbird whose main purpose is to sing and make people happy. Most of the children in
the novel are symboized as human mockingbirds--innocent kids who are forced to witness
the evil around them; Tom and Boo are other human mockingbirds, both innocent men
accused of crimes they did not commit.