OAKLAND -- No birds were killed in a wood chipper during a tree-trimming incident outside a downtown post office earlier this month, a U.S. Fish and Wildlife investigation has found.

Steve Martarano, a spokesman for the federal agency, said Tuesday that investigators made the determination after interviewing witnesses, reviewing a video and examining the contents of the wood chipper.

The finding confirms statements from Oakland police, the U.S. Postal Service and contractor Ernesto Pulido, who all said no birds were killed in the wood chipper during the May 3 trimming of trees surrounding the parking lot of the post office on 13th Street between Jackson and Alice streets.

Black crowned night herons are nesting in the trees that surround the parking lot across from the Oakland Civic Center Post Office in Oakland, Calif., on
Black crowned night herons are nesting in the trees that surround the parking lot across from the Oakland Civic Center Post Office in Oakland, Calif., on Wednesday, May 6, 2014. A contractor was hired by the U.S. Postal Service to trim the trees over the weekend causing great concern among naturalists and residents in the neighborhood. (Laura A. Oda/Bay Area News Group)

However, five black-crowned herons, estimated to be 1 week to 3 weeks old, were hurt after falling some 25 feet from the trees as workers hacked away at branches. Outraged onlookers rescued the birds and eventually took them to International Bird Rescue in Fairfield. Last week, a spokesman for the center said the birds were in "good condition" after suffering scrapes and bruises from the fall. One bird also had a fractured beak.

The U.S. Postal Service hired Pulido to trim ficus trees, where numerous black-crowned herons and snowy egrets have nested for years, to stop the birds from defecating on mail trucks parked below dozens of nests.

Investigators have not determined whether the U.S. Migratory Bird Treaty Act, a federal law designed to protect migratory birds, their nests and habitats, was violated. A violation of the act would be considered a misdemeanor offense that could result in a jail term as long as six months and a fine of a $1,000, plus $100 for any bird or nest that is harmed.

Pulido, of Bay Point, has expressed regret for the incident, and has offered to pay for the care of the five birds.

David DeBolt covers breaking news. Contact him in Richmond at 510-262-2728. Follow him at Twitter.com/daviddebolt.