Blue Jay

"The Blue Jay can be beneficial to other bird species, as it may chase predatory birds, such as hawks and owls, and will scream if it sees a predator within its territory. It has also been known to sound an alarm call when hawks or other dangers are near, and smaller birds often recognize this call and hide themselves away accordingly. It may also be aggressive towards humans who come close to its nest, and if an owl roosts near the nest during the daytime the Blue Jay mobs it until it takes a new roost. However, Blue Jays have also been known to attack or kill other smaller birds.[12] Jays are very territorial birds, and they will chase others from a feeder for an easier meal. Additionally, the Blue Jay has a bad reputation for raiding other birds' nests, stealing eggs, chicks, and nests. However, this may not be as widespread as is typically thought"-Wikipedia