A goose in Deering Oaks park displays signs of “angel wing,” a fatal malformation caused by unnatural diets. Courtesy Portland Parks, Recreation and Facilities

Geese in Deering Oaks park were spotted with deformed wings on Monday, Portland’s Parks Department said in a Facebook post Monday.

Some geese are suffering from a condition called “angel wing,” which is a malformation of their wings caused by an “unnaturally high-calorie diet (bread),” the department said.

The condition usually results in a twist in the last joint of a bird’s wing, causing feathers to protrude from its body, according to the Maine Audubon.

The condition, which is fatal, prevents the geese from being able to complete their usual migration patterns. It’s caused by a diet high in proteins and calories, but low in nutrients like vitamins D and E and manganese.

Another bird with “angel wing” at Deering Oaks park. Courtesy Portland Parks, Recreation and Facilities

“Angel wing” is usually incurable, and it undermines birds’ ability to fly, as well as the insulating properties of their feathers.

“You all can help by not feeding the birds in our parks,” the department said. “Please help us educate all those you know.”