The Eastern Bluebird has many predators. Raccoons, Squirrels, Opossums, Starlings, Humans, House Sparrows, House Wrens, Grackles, Blue Jays, cats, mice, snakes, insects, birds of prey, etc. It is important to try to keep these predators from reaching Eastern Bluebirds, their fledglings, and their eggs. Raccoons, opossums, cats, and squirrels will reach in a nest and take whatever they can get. Insects like Paper Wasps can kill young fledglings by stinging them.
Snakes can eat the fledglings and eggs whole.
Then, afterwards, they may decide to build over the old bluebird nest. Grackles and Blue Jays will take fledglings or eggs. Birds of prey can catch the adult bluebird or a newly fledged bluebird. Many of these problems can not legally be prevented, but some of the problems can. To keep House Sparrows and Starlings away from your Eastern Bluebirds you must monitor nesting sites at least once a week.