Most crows will take eggs and nestlings when they find them. They will also take small lizards, snakes, and rodents. Bluebirds are mostly immune to blue jays, because their eggs and young are in cavities or nest boxes. Blue jays bully other birds but not always with predatory intent. Blue jays also possess a repertoire basic to their species.
Individuals appear to learn additional calls.
I've never heard from one before. They do occasionally adopt all or a portion of a call from another species. The hawk then hopped down into the birdbath and drowned the sparrow, after which it flew off with its prey. We have never seen this before. T.M. Green, Lopez Island, Washington.