In slang, 'the bird' refers to the rude middle finger gesture used to show anger or contempt. It's most often heard in the phrase 'flip someone the bird' or 'give the bird'. It can be playful among friends, but it's generally offensive.
The bird a nest, the spider a web, man friendship. William Blake
The greatest achievement was at first and for a time a dream. The oak sleeps in the acorn, the bird waits in the egg, and in the highest vision of the soul a waking angel stirs. Dreams are the seedlings of realities. James Allen
Success is full of promise till men get it; and then it is last year's nest from which the bird has flown. Henry Ward Beecher
What is sport to the boy is the death of the bird. Hindi Proverb
Although the snake does not fly it has caught the bird whose home is in the sky. Nigerian Proverb
The bird that escapes from its cage never wants to come back. Vietnamese Proverb