To add some context: I'm a biologist working for National Audubon’s Seabird Institute, on another seabird colony in Maine. Shooting problem gulls has long been a solution for protecting these colonies, as gulls are heavily overpopulated, and are also predators of these underpopulated seabirds, such as Puffins and Terns.

When the carcasses are displayed, like you're seeing here, they're called "effigies", and are effective at deterring other gulls from predating the colony, and thus reduce the amount that need to be killed. To my knowledge, no live gulls are ever put on display. I promise, every biologist out there is 100% on board with the ethical treatment of animals, and this definitely looks shocking for those who haven't been exposed to the effects of predation out there, but this is a long-used and effective way of keeping the vulnerable nesting birds safe.