When tamed it can get fish and instantly kill them in Hunt and Retrieve, it will glow yellow and you can get Raw Prime Fish off it. Also it can hunt and instantly kill Lystro while on Hunt and Retrieve, and it will glow yellow and give some Raw Prime Meat. However, it can't do this with Dodos, at least not in Primative Plus, maybe in normal Ark it can do this with Dodos.
Using a Bola on one of these dudes is way easier said than done, I found it easier to shoot them with a tranquilizer arrow. I brought down a level 14 in one shot, after wasting 8 Bolas. Despite how annoying they can be. When their tamed they are extremely useful, for getting prime from fish and small animals, along with making taming otters and hespies much easier. If you set them to hunt and retrieve, you can target a fish or small animal and they will kill it and bring it back to you, so you can kill thoes salmon without having to step foot into the water. You can also use them to clear out piranhas. They are incredibly useful to have tamed, but very pesky when wild. One of the worst things to encounter early on is a flock of these guys. I usually see them alone, but I've also sewn them travel in large flocks and they will circle you and relentlessly steal everything edible from you, because unlike pegos that run off, the seagulls don't leave. But sometimes they don't aggro. I spent a whole hour trying to get this one bird to aggro on me, but he simply would not. Apparently whenever you come close to these birds it does a dice roll for if the bird will attack or not, and they always seem to aggro when you don't want then too, but never aggro when you want then to chase you so you can tame them, because 9 times out of 10 their flying above the water, however it's not too uncommon to see them inland if your exploring the southern islets region.
P.s. they will only eat fishy, but like most creatures they gain food from kibble. Not alot of people know this but you can literally just feed creatures kibble unless they have a really strict diet. So if you don't have any fish you can feed them a cheap kibble. If your playing on the switch, the seagull egg is used for bee kibble.