The best way to knock out your creature is to use tranq arrows if you are not that high of a level and you want to knock it out over land. Once you do that I hope you are close to water because you will need fish meat. I recommend that you don't bring fish meat but you just harvest it around your area because it could possibly spoil or you might not bring enough to which not bringing fish meat will help because while you are hunting fish meat then his hunger will go down and will make the tame faster.
Ever need lots of raw fish. Easy! Instead of swimming around in the water risking getting attacked by megapiranhas, simply make a saddle for your Pelagornis, Ride it into the water, and it one shots every small fish.