So these are actually useful. If you use it to attack dodos, lystros, fish and other small creatures it kills them and they glow. When they glow and you harvest them you get prime meat from land animals and prime fish from fish.
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.
In my opinion this is easily the best shoulder mount. It brings you fish that contain raw prime meat and sending it to attack a Dodo can get you raw prime meat. I have x1.5 harvest rate and got somewhere around 12 raw prime meat from one Dodo. An absolute must have