From the Super Mario Wiki
Ninja Crows are black crows with a red bandanna on their heads, encountered in the Wario World level Mirror Mansion. They are easy to defeat alone, but they almost always come in groups. After taking a few punches, Ninja Crows may produce a kunai from their mouth and jab Wario with it. Even when unprovoked, they can kick Wario with their sharp claws. Wario should stun them quickly and finish them off with a Piledriver, Wild Swing-Ding, or Power Throw. The crows are invincible while performing their stabbing attack.