What belief does Cassius express to Messala before the battle? How might this belief have influenced Cassius’s later decision to take his own life?

Cassius, who is superstitious, tells Messala that the omens that surround them portend doom.... the fact that it is his birthday unnerves him.... and the eagles that are replaced by crows are a symbol of death.

This day I breathed first: time is come round,
And where I did begin, there shall I end:
My life is run his compass.

...ravens, crows, and kites
Fly o'er our heads and downward look on us,
As we were sickly prey. Their shadows seem
A canopy most fatal, under which
Our army lies, ready to give up the ghost.