What omens and signs preoccupied Cassius on his birthday?

In Act V, Scene I, Cassius calls Messala over and tells him that the battle is similar to the one Pompey fought and lost against Caesar. He points out that although not normally superstitious, he is upset by the fact that two eagles who accompanied the army all the way from Sardis have been replaced by crows and ravens, symbols of bad things to come. He ends his speech by stating, "Our army lies ready to give the ghost" (5.1.88).