The Encounter (Animorphs)

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 its four kids and a hawk against a force that is determined to destroy them or die trying