"Around 15 young men, aged between 20 and 25, were catching the birds, throwing them in the air, using birds which were still living as soccer balls, and tearing off their heads," Razvan Popescu, director of the Oceanic Club wildlife protection organisation told Reuters.
The birds had been too cold and tired to fly as temperatures plummeted as low as minus 20 Celsius near the Danube delta.
"There were up to 100 migratory birds, ducks, geese and coots," Popescu said.
He said the Oceanic Club had been doing checks on migratory birds as the lakes and rivers froze when they saw the men playing with the birds.
Veterinarians said samples were taken from some of the dead birds and tested for bird flu. They came out negative.
Romania has found avian flu in poultry in 26 villages since October, but has recorded no human cases.
Health Ministry officials said doctors were now monitoring the men because many of the people who have contracted bird flu worldwide had had close contact with infected birds.