I realised how to kill this guy!

Here are the steps:

1:Tame Argentavis

2:Mock them cause they can't fly

3:Whistle Gigas to kill him cause he is now aggro onto you

4:Shoot tranqs and finish the job!

WARNING:Killing one turkey may cause others in the area to aggro on you so it's better if you are alone with turkey.

Oh btw I think it's obvious that you fly and perch on a tree or stay in the air!