I once came across a study in my Sociology class.
Interestingly, the bird involved in that study was the same as above, I think its Blue Robin. The study was about the mating habbits of Blue Robin. The researchers placed a Dummy/Fake blue robin near a place where Male Blue Robin would visit his Female Blue Robin.
And they noticed that the Male Blue Robin would show the signs of Aggression and would BEAT the Dummy Blue Robin, as it considered it its COMPETITION. Once the Mating Season was finished, and the Female had laid the eggs, the Male Blue Robin, stopped his Aggressive behavior toward the Dummy/Fake Blue Robin.
Maybe, in this picture, this blue robin, is in the Mating Season, and when he spots this another Blue Robin, I the DEAD one is also a Male, he is trying to aggrevate him, to FLY AWAY, and GET AWAY from Nesting Area, where HIS Female Might be nesting.
So, we don't know the exact story.
The Birds, particularly, the CROWS are known to show some emotions. In Pakistan I remember, if a Crow Died, the whole Flock of Crows would sit near and around the tree, and if some kid kicked that dead bird, or tried to move it, the other birds would get aggressive and would go after that KID, and try to strike him on his head with their beaks..
LoL..