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The dead robins are back.

More than 30 carcasses have been found on the ground in Northeast Portland in the past week. Wildlife experts don’t know for sure what killed them but one possible cause is a berry binge — just like in February 2008.

That month, the carcasses of more than 50 American robins were found around Mount Tabor in Southeast Portland. When scientists opened them up, they found their bellies full of holly berries.

“They had gorged themselves on fermenting berries,” said Bob Sallinger, conservation director at the Audubon Society of Portland.

The robins had died of alcohol poisoning.

“If robins eat enough fermenting berries, it’s like binge drinking,” Sallinger said. “It’s not like long-term liver damage.”

Residents found the bodies this week near Northeast Russell and Rodney. They contacted the Audubon Society, but their wildlife experts found no obvious cause of death so they turned them over to the U.S. Fish and Wildlife Service, which sent 10 carcasses to its forensics lab in Ashland for analysis.

The test results might not be available for four weeks. In the meantime, Sallinger is keeping an open mind.

“It could be poison,” he said. “It could be that there’s a disease running through the population. There are a lot of things that can kill birds.”

But he acknowledged that the two dieoffs are similar. “There are some interesting parallels,” he said.

Both times, the robin carcasses appeared normal, with no visible signs of injury or disease. And both times, the birds died after a cold snap.

American robins, one of the five most common birds in Portland, prefer to eat worms. But when the ground freezes, they’re forced to consume what’s available.

“If worms are scarce, they will go for things like berries,” Sallinger said. “That may have been what happened here.”

A resident discovered a few more dead robins Friday morning near Northeast Russell and Rodney. There are no holly bushes at the intersection, but a few are mixed into the landscaping of a nearby apartment complex.

Most of their berries are gone.