I don't know whether this'll work with trees but when we sowed a new lawn my Dad (who knows this kind of stuff) said to tie black cotton across. The birds can't see it well but fly into it and get freaked out so stay away apparently! Might be worth a try.
Get some of those whirly windmills, hang CDs in the tree! But don't worry too much - as crazyoldbaglady says, a tree can lose a lot of blossom before the crop fails. The number of flowers greatly exceeds the number of apples the tree can possibly bear.