Bola it, then use a slingshot to body shot it. Don't head shot it; it'll die within 5 hit of the slingshot. I used a crossbow and two headshots and dead. Use fists, sling, or boomerang to knock it out. Good luck, 2 2.5x Coels should suffice for the taming.
They need to make it so that if you put narcos bio toxin etc in ur last hot bar a lot the icthyornis will eat them and get knocked out(like putting berries in ur last hotbar slot to tame the pego, but instead of taming it will be knocked out)
Similarly to real life, these greedy douchebags will try to steal your sandwiches, however if you are drunk enough, you can stay up into the late hours of the evening and have deep insightful conversations with these feathered friends as they simply stare at you in confusion and bark in bird language. In return they will bring you magical fish as a reward.
The way I tame mine was I had bolas in my hotbar and about 3 bolas just in cause. After I knocked it I put raw/fresh fish meat on it. Then I went out to the ocean and I killed some sharks with my thyla and hachet the sharks to get raw/fresh prime fish meat.( I was on Valguero and I was playing Primivite+.