In her fourth-floor apartment in central Rome, Emanuela Tripi awoke at dawn to the terrifying sounds of a home invasion. She crept into her kitchen and spotted the culprit long white neck, red-rimmed eyes, yellow-webbed feet stabbing its beak into a garbage bag. Growing up in Sicily, Ms. Tripi always had a romantic vision of sea gulls, but now she was face to face with a predator that has aggressively colonized a city a good 20 miles from the sea. She threw a slipper. It ignored her. She lifted a second slipper. It cawed violently, took flight and charged. "Arrivederci, you win," she thought as she ran out of the kitchen, closing the door behind her. As her cat cowered, wetting the couch, she banged on the door to scare off a bird she described as "enormous, above my knee, as big as an American wild turkey." But it stayed "like it was its place" she said, until it was done eating and flew back out the window. Romans have for years bemoaned the degradation of their city: the potholes, the burning buses, the unkempt parks and the uncollected garbage, stinking its streets and clogging its river. But the sea gulls aren't complaining about the overgrown spaces and free food, and their raucous sundown ritual of circling over the Forum and Palatine Hill does not augur well for Rome. "We've told them Rome is their home," said Francesca Manzia, the director of the Italian League for Bird Protection in Rome. "And they are acting like it." The sea gull population in Rome has grown in recent years to the tens of thousands, according to some experts. Their physical dimension has grown, too, as they gorge on the smorgasbord of trash, snack on handouts from complicit tourists and snatch sandwiches from unsuspecting pedestrians. A species with a taste for pigeons, bats, starlings and sometimes other sea gulls, the Larus michahellis protects its territory like a local heavy. If some mayors promise a chicken in every pot, Rome's have delivered a sea gull atop every garbage bin. The birds, which can live for decades, have settled comfortably in the city's rooftops, church towers and ancient ruins. Their shrill squawks, sounding like a flock of colicky babies, pierce the evening sky. "Once in contact with a new species us they have learned to respond," said Ms. Manzia, who explained that sea gulls interpret human handouts as a sign of submission. "They think, 'O.K., this is my territory now.' " With no culling plans in the works, Ms. Manzia said she has repeatedly explained to city officials they need to clean the city and improve Roman behavior if they want to reduce the sea gull population. "The city says, 'Impossible,' " she said. And so, at a recent performance of "La Traviata" in the Baths of Caracalla, an ancient ruin and longtime sea gull haunt, I listened as an operatic duet became a trio with a screeching sea gull. On a rooftop outside the Vatican, where sea gulls have ripped to shreds peace doves released out the pope's window, I watched as a pair ominously swooped above the purple zucchetto, or skullcap, of the Vatican foreign minister. A couple in the Trastevere district, with a terrace to die for, risk, well, dying on it, as squatting gulls defend it beak and talon. This avian onslaught has spurred a resistance. Barbara Nat, an architect, saw the birds shredding the garbage bags under her balcony by the Circus Maximus and grabbed three oranges from the kitchen. She fired them at the birds and connected. "It felt good," she said. Ms. Nat, like many Romans, is convinced the Roman gulls have mutated into monstrous proportions. But experts insist they seem bigger only because humans are not accustomed to seeing them up close. Not everyone is buying it. "That's not a sea gull," shouted Joe Potenza, as he sidestepped two large birds fighting over pizza scraps on the Bridge of Angels. Mr. Potenza said the sea gulls in his native Melbourne, Australia, were half the size of their Roman cousins, which are big enough, he worried, to "bite your hand off." For thousands of years, the she-wolf that suckled Romulus and Remus, the city's mythic founders, served as the symbol of Rome. But in May 2017, the administration of the oft-maligned mayor, Virginia Raggi, graced its official Facebook page with a photo of a sea gull standing triumphantly over the Forum. Residents responded angrily. "Thanks for giving a lousy image of a city that is always more mired in ugliness," read one comment. The administration took down the post, apologized and substituted it with its procedures "to reduce the sea gulls in the city." But the sea gull, while taking a post-breeding season breather in the late summer and fall, doesn't seem to be going anywhere, and like the she-wolf before it, has its own mythic origins. "It's a long story and it involves me directly," wrote Fulco Pratesi, a well-known wildlife advocate, on his blog in 2009. Legend has it that in 1971, a friend of Mr. Pratesi rescued an injured female gull from a Tuscan island and brought it in a shoe box to his office at the Rome zoo. Mr. Pratesi built a home for it in the seal pool and then one spring the injured bird "attracted a wild male gull flying over these parts." The birds coupled and created a nest out of garbage. The rest is history. Ms. Manzia was incredulous. "It's like we don't come from Adam and Eve," she said. The arrival, she said, was actually the result of foraging gulls that followed the Tiber River to Rome, and then spread word of the city's bounty to other birds. At first, the Malagrotta landfill outside the city, Europe's largest dump until the authorities ruled it unfit to treat waste, drew the gulls in droves. Since its closing in 2013, Rome's uncollected garbage has picked up the slack. The Vatican has added rare delicacies to the menu. In January 2014, during a Sunday prayer for peace in Ukraine, two children flanking Pope Francis at his window in the Apostolic Palace released two white doves. The pope wished everyone his customary "good lunch," and a sea gull, aided by a hooded crow, obliged. "Oh God, that was a nightmare," Ms. Manzia said. "They called me from all over the world asking, 'Is this a bad omen?' " Ms. Manzia said she had repeatedly told Vatican officials that the domesticated white doves are easy prey for the gulls and that traveler pigeons should be used instead, which at least have a chance of fleeing, but the Vatican has stuck with the doves. "It's the Vatican's own fault," she said. "They say they have a different state with different laws." For the sea gull there is only the law of natural selection. Despite all the bad headlines they have attracted ("Seagulls Attack Man," "Seagulls Injure Child"), and all the internet videos of sea gulls feasting on a pigeon atop a police car or dragging rats around piazzas, the birds can't be blamed for following their instincts to protect their young and enjoy the all-you-can-eat streets. But it's hard to forgive them their smell. "Being an animal that eats garbage," Ms. Manzia said as she walked out of a pungent cage filled with dozens of sea gulls, "they stink like garbage."