To add to this, songbirds also sometimes engange in mobbing when predators show up. Different species will all flock together and bombard the predator. I've watched it happen to my cat before when he was chasing chickadees around. All the birds in the yard ganged up on him and chased him off. He's not a very good hunter, so the birds weren't really in danger either way. One of my biology teachers mentioned if you make the right noises while walking through the forest, you can trick the song birds into thinking you're a predator and see the phenomenon for yourself.