The Encounter (Animorphs #3) by K.A. Applegate

When Tobias, Jake, Rachel, Marco, and Cassie were given the ability to morph, they were also given one very important warning: Never stay in a morph for more than two hours. It seemed a small price to pay, since the kids know that humans everywhere are being forced to let slimy, spineless creatures creep into their brains. And the only way the kids can fight back is not to be human.

But Tobias stayed in his morph too long. And now he's a hawk -- with a boy's mind -- forever. Tobias knows they can't give up. That they all made a promise. So now it's four kids and a hawk against a force that's determined to destroy them. Or die trying.