In the ruins you passed through in the prologue, the vultures have set up shop. As in most lairs, your objective is to kill the highest ranked beast among them, which Senu can help you ID.
A predator bow is ideal for this type of quarry- if you don't have one yet, there is a lootbox toward the south end of the ruins that may contain one.
If you don't have the bow for it, or run out of arrows, you can just get in the vulture's face enough until it starts attacking you, at which point it can be melee-killed.
Whatever your methods, killing the vulture king ends the mission.