These large birds are what dinosaurs used to be. (no not Trex but Raptors) Look at wild turkeys, if they are in a group they can fuck you up. Most times they will run scared but they can and will attack if they are force to fight.
This is a wild turkey skeleton, you can easily see it's similarities to certain types of dinosaurs like the Raptor as example.
Many of the smaller dinosaurs evolved into smaller birds as the larger ones went extinct.