In the film, they are portrayed as mindless eating machines who only shout one word: "Mine!".
They were first seen when they were cawing "Mine!" while Nigel had a crab in his wing that he was going to eat, much to his irritation. However, he ended up relieving the crab when he overheard from his fellow pelicans that Nemo's father, Marlin, was trying to search for his son, although the crab managed to escape from the Seagulls.
Later in the film, the Seagulls surround Nigel, Marlin and Dory, and also nearly try to eat Marlin and Dory while trying to escape from Nigel (as they thought he was going to try and eat them when all he was trying to do was tell them that he knows Nemo and where he is), but Nigel saves them. They chase after Nigel until their beaks crash into a sail of one of the sailboats.
The Seagulls made their last appearance when Dory and Nemo were trying to get information out of a crab about where he saw Marlin go (Dory intended to feed the crab to the Seagulls if he didn't answer them).
Disney Parks and Resorts
Three of the seagulls appear on top of a buoy in the Finding Nemo Submarine Voyage lagoon. The periodically flap their wings and say "Mine, Mine, Mine." They appear as Audio-Animatronics voiced by Andrew Stanton who was the director of the film.
Three of the seagulls reside on the rocks outside the entrance to The Seas with Nemo & Friends at Epcot. Like their Disneyland counterparts, they periodically flap their wings and say "Mine, Mine, Mine."
At Finding Nemo: The Musical in Disney's Animal Kingdom, they don't actually appear in the show itself, but they do appear in the bubbles flanking the stage after the show ends to say "Bye, Bye, Bye."
Characters: Nemo | Marlin | Coral | Dory | Pearl | Tad | Sheldon | Mr. Ray | Bruce | Anchor | Chum | Gill | Peach | Bloat | Bubbles | Deb | Gurgle | Jacques | Nigel | Crush | Squirt | The Dentist | Darla Sherman | Moonfish | Barracuda | Anglerfish | Seagulls | Jellyfish | Whale | Chuckles | Blenny