The most likely reason that a bird would develop a long beak in a temperate forest is because it feeds on insects and larvae that live in the trees. This kind of beak, long and thin, is necessary for the bird to be able to get to its foo source. It will either look for a whole in the tree, or it will make a whole with its beak, and than use it to penetrate deeper into the hollow part of the tree and get the insects or larvae. This can easily be seen in the woodpecker, which is a prime example of a bird with that kind of beak and that kind of a feeding habit.