They have been a familiar sight around the lake in Prospect Park in Brooklyn: Canada geese, scores of them. To some residents, the birds and their fuzzy offspring are charming hints of wildlife amid the bricks of the city. Recently, when one was found with an arrow through its neck, park rangers tried to corral it to administer first aid.

But then, over the last few days, parkgoers noticed something strange.

The geese were gone. Nearly 400 of them.

On Monday, the answer emerged. Wildlife biologists and technicians had descended on the park Thursday morning and herded the birds into a fenced area. The biologists, working with the federal Agriculture Department, then packed the geese two or three to a crate and took them to a nearby building where they were gassed with lethal doses of carbon dioxide, Carol A. Bannerman, a spokeswoman, said.

Ms. Bannerman said the measure was necessary. “The thing to always remember in this New York situation is that we are talking about aviation and passenger and property safety,” she said. “In New York City, from 1981 to 1999, the population increase was sevenfold.”