Here's how I tamed one using Ichthyornis. πŸ‘‡

What I did was use my Ichthyornis to Hunt fishes and bring the dead bodies for me.

Change his settings to "Hunt and retrieve", put him on your shoulder then you go swim. While you are swimming aim at The fish you want and double tap F (on PC). Ichthyornis will fly, kill the fish and bring the body to you.

Then you take the body to Hesperornis and feed it. Once you get used to it it's very fast and easy. I recommend following Hesp from behind so you dont need to adjust your aim all the time.

You can do the same to tame Otters.

More Hesperornis Taming & KO Tips