NABU's Berthold Langenhorst said around 18,000 cranes had been seen in Hesse, on their way up to northern Germany where they will nest.
The first groups are thought to have arrived from Spain and southern France. Generally they start this migration at the end of February, but experts say the sudden change in the weather may have sparked off an earlier departure.
After months of freezing weather, things seem to be thawing out across central Europe, including Germany.
The forecast is for stormy weather across the country on Sunday, after a Saturday of changeable weather, including patches of sunshine.
Temperatures are expected to reach a maximum of six degrees in the north east, and as high as 13 degrees elsewhere.
The start of the new week could see some rain and even snow in the northernmost areas, with stormy and showery weather.