As I write there's a tremendous racket going on in my neighbor's back yard. The blue jays are screaming and jumping around in the spruces, focused on a spot I cannot see. Starlings are loafing nearby to see what happens. Twelve nervous mourning doves are sitting in the black locust. The juncoes have joined the fray in the spruces, even though they're normally wary of jays.
Everyone's excited about a predator in the spruces. I can only guess it's an eastern screech-owl.
Blue jays are an excellent alarm system. I often use their calls to find birds of prey. Small birds are saved the surprise of being eaten when they hear the jays' alarms. Even though the rest of us benefit from them, the jays just view it as doing their part to make the world safe for blue jays.