There was nothing mysterious about the death of a flock of birds in Romania last week -- they were simply drunk, veterinarians said.

Residents of the Black Sea city of Constanta alerted authorities on Saturday after they found dozens of dead starlings, fearing they may have been infected with bird flu, which triggered mass deaths in avian populations in 2004-2006.

"Tests on five birds showed gizzards full of grape marc which caused their death," Romeu Lazar, head of the city's veterinary authority told Reuters, referring to a pulpy residue which is a by-product of winemaking.

"This also applies to two dead crows we tested," Lazar said. Birds are not used to alcohol but harsh winter and snow had prevented birds from finding food. Had they been able to eat some seeds, this would have diluted the poison."

The grape marc was presumed to have come from a winery, but the veterinary chief said he did not know where.

There have been a series of unexplained mass bird deaths in several countries across the globe in the last few weeks, including in the United States and Sweden.

Hundreds of dead birds were discovered in Louisiana this month and 5,000 in Arkansas at New Year. Swedish authorities have also been investigating the deaths of 100 jackdaws found in a street in Falkoping. Experts say storms, hail, lightning or collisions with airplanes or power lines are among the possible causes of bird deaths.