What is the symbolic significance of the big yellow bird?

Basically the yellow bird was an imaginary friend to Perry that came to him in his dreams,

"Throughout his life – as a child, poor and meanly treated, as a foot-loose youth, as an imprisoned man – the yellow bird, huge and parrot-faced, had soared across Perry’s dream, an avenging angel who savaged his enemies or, as now, rescued him in moments of mortal danger." (Capote 266)

Although Perry did not consider the bird to be Jesus, it remained a salvation-figure in his psyche.