Freedom and captivity. Birds can fly away but they can also have their wing feathers clipped or be kept in a cage. Gilded or otherwise.I believe that the symbology of the bird is predicated upon its use by the author.
In the Book To Kill A Mockingbird by Harper Lee, I believe the mockingbird symbolizes Tom Robinson, the black man on trial that Atticus defends. Tom is accused of raping a white woman. He is innocent but found guilty. He tries to break out of jail and is shot and killed by one of the prison guards.
Atticus tells his daughter Scout that, “it’s a sin to kill a mockingbird”. A case could be made that the mockingbird symbolizes innocence and it’s loss. Interestingly, Atticus’ last name is Finch.
In the book The Goldfinch by Donna Tartt, the plot revolves around the painting The Goldfinch by Fabritius. The bird is shown sitting on a perch. But if you look closely you will see a string around its one leg, tethering it.
The bird only has the illusion of freedom. As do the main characters in the book.
Women through the ages were like birds in gilded cages. Once married a woman and anything she owned became her husband’s property. She was completely dependent on her husband for everthing. Like the Goldfinch painting, this seemed perfect at first glance, but was only illusory.
Crows have always symbolized death or were harbingers of evil. Their collective noun, a murder of crows, says it all. Doves historically signified peace and purity. Though mourning doves, with their plaintative song, signify sadness. Swans signify love. Phoenixes, though mythical, represent rebirth. Owls represent intelligence. Robins announce the coming of spring. Canaries let us know we can breathe in a mine. Songbirds sing and bring us joy.
These are just a few interpretations of birds and their symbolism. Every writer has their own idea of what a bird represents. The only limit is their imagination.