We think of the Blue Jay as a community warning system. If there’s gaggle of Blue Jays all raising an alarm call, chances are good there’s a hawk or other predator around. Sometimes they are warning of a Cooper’s Hawk sitting in a tree waiting to pick off its next meal and sometimes they may be telling the other birds about a Red-tailed or Red-shouldered Hawk flying high.
We’ve noticed that Blue Jays will also mimic the call of hawks, especially the call of the Red-shouldered Hawk. Nobody is quite sure why they do this. Maybe it’s a signal that hawks really are around or maybe it’s a deceptive ploy to run everybody else off. 
The Blue Jay is a striking bird. It’s predominantly blue with a white chest and underpants, and a blue crest. The Blue Jay has a black, U-shaped collar around its neck and a black border behind the crest. Both genders are similar in size and plumage, and plumage does not vary throughout the year. 
The blue Jay is also very intelligent.  Young individuals playfully snatch brightly colored or reflective objects, such as bottle caps or pieces of aluminum foil, and carry them around until they lose interest. 
We’ve always thought of the Blue Jay as a very aggressive bird, and in certain situations they are. However, Blue jays are frequently subservient to other medium-sized birds who visit bird-feeders. In Florida, Blue jays are dominated at feeders by Eastern gray squirrels, Florida scrub-jays, common grackles and red-headed woodpeckers, all of which have been occasionally observed to aggressively prevent the Blue Jays from feeding. 
Blue jays are not very picky about nesting locations. If no better place is available, they will even use places like the large mailboxes typical of the rural United States. They also appropriate nests of other mid-sized songbirds as long as these are placed in suitable spots; American Robin nests are commonly used by Blue Jays, for example. 
References and Additional Information
 Wikipedia: Blue Jay
 The Sibley Guide to Birds, second edition – David Allen Sibley
 Cornell – All About Birds: Blue Jay
 Cornell – All About Birds: Blue Jay – sounds
 The Breeding Bird Atlas of Georgia – UGA Press: Schneider, Beaton, Keyes and Klaus, Eds.