What is the chance happening that precipitates the catastrophe?

A storm has arisen. Then Cassius reveals to Messala that it is his birthday and adds that he has never truly believed in omens. But as he was arriving he witnessed to eagles being fed by his troops, then the next day the eagles were gone and had been replaced by ravens, crows, and kites. These are all carrion feeders and looked upon as an omen of death.