It's easy to miss the way that Pound repeatedly mentions sea birds in "Canto II." That's probably because they don't play any direct role in the action. Instead, they seem to be these weird witnesses or bystanders who don't really care about all the magical, epic stuff that's happening around them. In other words, they represent nature for Pound, since nature doesn't really care one way or the other what happens to human beings. Sorry, but it's kind of true. When was the last time you fell down and a nearby seagull expressed concern?
Lines 29-33: While Poseidon is raping the nymph Tyro underneath the cover of a water tent, a bunch of gulls "broad out their wings,/ nipping between splay feathers" (29-30) as though nothing worthwhile is going on. If there were some fish nearby, they'd probably care. But the nearby sexual assault means nothing to them. Similarly, some other birds called "Snipe" fly around and "come for their bath" (31) out in the ocean. But again, they couldn't care less about the human drama happening nearby.
Lines 140-141: The "Sea-fowl" pop up again toward the end of "Canto II," but only for the sake of "stretching [their] wing joints" (140) and "splashing in rock-hollows and sand-hollows" (141). Their meaning hasn't changed at all. They're like a refrain, with their indifference always being in contrast with the epic human drama unfolding close to them. What Pound is basically saying here is that all human drama unfolds against the background of nature, which really doesn't care about humanity all that much.