Bird droppings were the likely cause of a December shutdown at a nuclear power plant outside New York City, according to the operator.

An Indian Point reactor safely shut down for three days starting on December 14 following an electrical disturbance on outdoor high voltage transmission lines, Entergy Corp said. An outside expert is analysing whether what is technically called bird "streaming" was the culprit.

In a report to the Nuclear Regulatory Commission (NRC) last month, the New Orleans-based company said the automatic reactor shutdown was apparently from bird faeces that caused an electric arc between wires on a feeder line at a transmission tower.

"If it has nowhere to send its electricity, the generator senses that and automatically shuts down," Entergy spokesman Jerry Nappi said.

Plant managers told the NRC they were revising preventive maintenance for additional inspection and cleaning, and installing bird guards on transmission towers.

Mr Nappi said he could not recall a similar incident in the past several years from birds at Indian Point, which is located along the Hudson River north of New York City.

NRC spokesman Eliot Brenner said it is not uncommon for wildlife to trigger electrical outages on transmission lines regardless of the generation source of the electricity. "Squirrels are the biggest offenders," he said.

He did not know if the NRC was specifically tracking animal-related reactor outages. "They're kind of few and far between, but they're not uncommon," he said.