OAKLAND, Calif. — It started as a well-intentioned attempt by the United States Postal Service here to rid its trucks of bird droppings: A tree trimmer was hired to prune the lush ficus trees that grow next to the post office’s parking lot, not far from City Hall. But in the course of the job, five baby black-crowned night herons fell from their nests and were injured.

At first there were reports that the birds had been fed into a wood chipper — not true — and from there the story took on a life of its own. Residents and city officials called for avian justice. Bird lovers from France, Romania, Serbia, Sweden, Ukraine and even New Jersey signed an online petition with the headline “Oakland Chainsaw Massacre” that called on the California Department of Fish and Wildlife to press charges against the perpetrators.

They have gotten their wish and more: The tree trimmer, Ernesto Pulido, 26, is staring at a possible federal charge of violating the Migratory Bird Treaty Act of 1918.

“I’m not a gangster,” said a contrite Mr. Pulido, who has visited the bird shelter where the victims are recuperating and voluntarily paid $2,500 for their medical care. “I’m just a guy who’s making a living day to day.”