They flew over 2,000 miles (3,218 km) in just two days. It was possible due to high-pressure systems along their journey that provided a strong lift for the flocks.
Experts estimate that this huge flock comprised at least 9 million birds! The mass migration was so large that it could be seen on weather radar maps as a big dark blob stretching across Europe.
Scientists believe that group migration gives birds a better chance of survival since there is greater protection from predators when flying in larger numbers.