Because of the aggressive behavior of the Argentavis. It is quite easy to get there attention and to knock them out. Attack them once with whatever you have and then run to your Base or taming pen. The Argentavis will follow you the whole way. After u trapped him in your taming pen, you can start shooting tranqualising darts or arrows at him. Once he's knocked out feed him and place the saddle I the inventory. Once he's tamed you can ride him. The Argentavis will have around 300-400 carry weight so be careful that ur not to heavy or else u can sit on him but can't move in any direction. Like that you would be an easy pray for any Predator.

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