E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Bird in thy Bosom.
Thou hast kept well the bird in thy bosom. Thou hast remained faithful to thy allegiance or faith. The expression was used by Sir Ralph Percy (slain in the battle of Hedgly Moor in 1464) to express his having preserved unstained his fidelity to the House of Lancaster.