Blue jays are carrying sticks, building nests as high as they can get in canopy trees. While nesting, they are especially pugnacious around screech-owl cavities remembered from years past. They'll even fuss at new owl boxes in new spots, but they never mob chickadee or titmouse-sized boxes. Obviously, they've learned or instinctively know the potential for danger associated with larger cavities. By mobbing, I believe older individuals warn and instruct younger ones and inadvertently teach avian compatriots. Blue jays rank fourth on the screech owl's grocery list of birds but first in frequency of mobbing. Jays themselves are predators on the eggs and young of the same songbirds that may join them in mobbing screech owls. And predator-prey relations may extend to two hawks, which the jays mimic in the right season. By mid-February they quit sounding like redtails who wintered here but left to nest in the open countryside, and they don't begin to sound like broadwings until they arrive from the tropics in about two weeks. Broad-winged hawks will nest and leave in mid-August to mid-September, after which the jays mostly forego hawk calls until redtails arrive again in December.
Blue jays must achieve something by mimicking the correct hawk, but I have no good evidence of what it is. Our nesting broadwings do catch a few birds, but I haven't seen them with a jay, and I don't know about redtails eating local birds, only rodents and snakes. Does the vocal mimicry deter predation, because hawks believe that another hawk already has that territory? Does the mimicry disperse other predators or potential competitors? Blue jays avoid the area of last year's broadwing nest, and, since they begin nesting before broadwings return, memory must serve them as well as it does with screech-owl nest cavities.
Predator-prey relations include American crows that are also building nests high in plateau live oaks and eastern red cedars with evergreen foliage to hide them. Both crows and jays are very secretive around their nests. One neighbor had a crow nest ten feet above her front door and never knew it. Crows may have learned to be that way for successful suburban nesting, because they are quite bold otherwise, casually feeding in open lawns and prospecting for bird nests. Crows eat bird eggs, nestlings, and fledglings, including those of blue jays, so they're greatly mobbed by the jays who are, after all, just small brightly colored crows.