What scifi book has humans as the ultimate warrior species being recruited in an interplanetary war?

There were bird-people who specialized in diplomacy and language, rat-people who were formerly the front-line soldiers, and a whole slew of other semi-anthropomorphic species. Humanity entered the scene and started replacing the other species as the "warrior race" because of innate bloodlust. I can't think of the title or author, but I can remember the cover! It's frustrating!