What does Cassius confide to Messala before the battle?
In Shakespeare's Julius Caesar, before the battle Cassius reveals to Messala that it is his birthday. Cassius says:
This is my birthday: as this very day
Was Cassius born. Give me thy hand, Messala:
Be thou my witness that against my will
(As Pompey was) am I compelled to set
Upon one battle all our liberties. (Act 5.1.70-75)
Cassius is regretting that he has to set defeat or victory upon one, single battle. He compares himself to Pompey, who had to do the same against Caesar, and lost. He asks Messala to be his witness that he is doing this against his will.
Eventually, of course, Cassius's birthday will also become the day of his death. His fear becomes reality as the one, single battle is lost, as are all their "liberties," including Cassius's life.
Cassius is superstitious about the battle and its outcome. He tells Messala that he is nervous to fight the battle as it is his birthday. He believes he has witnessed signs that indicate that they are doomed to lose this battle. The eagles that once perched are now replaced with more sinister birds, such as the crows and ravens. These birds are more predatory and vulturous, whereas the eagle was noble and majestic.
Cassius wearily accepts that he will face whatever comes and prepares to battle. He and Brutus discuss their refusal to be taken back to Rome as captives. They agree that suicide is preferable to the humiliation of being captured alive.