“This has nothing to do with the mute swans. Somebody who has real political influence doesn’t like them, in my opinion, because there’s no real evidence here that they’re an invasive species or why they have to come up and kill them all,” Mr. Avella added. “Someone, some political entity that has political connections, is exerting some huge influence to get the agency to kill them.”

It was the second time in two years that Mr. Cuomo had vetoed the bill, each time explaining that it duplicated efforts by the environmental agency to revise its swan-management plan in the wake of the public outcry.

The plan, which originally called for eradicating virtually all 2,200 of the state’s mute swans by shooting, gassing and other measures, was changed this year to emphasize non-deadly control methods. The revised proposal, however declawed, failed to appease animal-rights activists and the bill’s sponsors, who continued to insist that the agency had presented no convincing evidence that the swans were harmful.

“Impartial observers find it impossible to really assess what the D.E.C. is accusing the mute swans of, because they haven’t provided any specific data, just these very broad, speculative accusations that may or may not be true,” said David Karopkin, the founder of GooseWatch NYC, an animal-rights group. “I’m not really sure it should be called science, because it doesn’t seem scientific to me,” said Mr. Karopkin, who added that he was not a scientist.

After the revised plan was announced, the Department of Environmental Conservation received about 8,000 comments, the vast majority of which were form letters expressing opposition, said Jomo Miller, a spokesman for the agency. It hopes to issue a final version of the plan by the end of this year, he said.