Meteorologists expect temperatures to drop below zero in coming days, threatening colonies of Dalmatians Pelicans, Pygmy Cormorants and Spoonbills in the vast marshlands of the delta, one of the most biodiverse regions in Europe.
"Some species, which were supposed to come at the end of February, are already here," Dan Hulea, president of the Romanian Ornithological Organisation, told Reuters.
Some birds are returning early to the delta, which lies on a migratory route for wild birds, and many never left when temperatures rose as high as 20 Celsius (68 Fahrenheit) earlier in January.
"If cold weather persists for more than 10 days, it will have a serious effect on the population if those birds start nesting, said Hulea. The population of young birds could drop by 20 percent, he said.
The delta, a UNESCO World Heritage Site since 1991, is home to more than 300 species of birds.