After the parley with Octavius and Antony in act 5, scene 1, Cassius speaks to his friend Messala and shares his apprehensions about the impending battle. Saying that he now takes omens more seriously than he used to, he is concerned that the two eagles that have accompanied his legion have unexpectedly left. Now, numerous ravens, crows, and kites follow his army.
Shakespeare was almost certainly well aware of Roman religious practices in reading the flight and activity of birds as a form of fortune-telling. Known as augury, Romans frequently looked for meaning in the behavior of birds as signs of what was to come and the will of the gods. Here, Cassius interprets the sudden departure of the eagles, symbols of Rome itself, as the possibility that fortune has turned against him and his army. Furthermore, the new birds that follow his soldiers and "downward look on us / as we were sickly prey" seems quite upsetting since ravens, crows, and kites are carrion-eaters and often associated with death and bad fortune. Cassius fears that his soldiers may soon become food for these birds. Despite these frightful omens, Cassius and Brutus remain determined to see the battle through and fight to the death if it comes to it.