The worldwide migration pattern of White Storks: differences and consequences
Humans have always wondered where the birds migrate to in winter. The first piece of evidence that storks really spend the winters in Africa was a spear that a stork brought back with him almost 200 years ago. The spear barely injured the stork and it was able to return from the hunting grounds to its breeding grounds with the souvenir.
The White Stork is an excellent species to study bird migration. Its appearance, its size and its deep roots in many cultures make it recognizable. In Europe it brings newborns, in Mesopotamia it marks the beginning of the pilgrimage to Mecca, and it often brings good luck or is even seen as holy bird. It doesn’t hide in the thickets, instead it parades on the meadows, builds nests near humans and its black and white plumage is easily recognizable.
Beginning in 1901, bird ringing made it possible to scientifically track the storks’ migration pattern. An extensive dataset was created and showed us that the European storks migrate on eastern and western routes to the African continent. The storks in southwestern Germany migrate to Spain through Gibraltar to Senegal while the birds from eastern Bavaria and Brandenburg cross the Bosporus and follow the Nile valley to reach Sudan and partly to continue to East and South Africa. Bird ringing can answer questions regarding age, distance between breeding location and location of birth, fidelity, and mortality rates in various years and regions. However, new questions were instantly raised: How intensively are areas used where no one observes or reports storks? Does migration behavior change throughout life? How much energy is needed to follow one or the other migration route. Is it more advantageous to migrate alone or in a group? How do they find their way? Why do single individuals diverge from the “common” migration habits and what are the consequences? What are the threats for the migrants in the 21st century? How do changes in land use and climate affect population size and migration behavior?
Thirty-three out of a hundred stork nestlings marked with a ring are found or observed later in their lives, on average they create 3 data points. This is an impressive rate for a wild bird but still too small to answer a series of questions. Scientists wanted many more data points about the localizations of the birds. In the 1960s, storks were painted in different colors in special studies to raise their reporting probability. White Storks were also among the first birds ever to carry a transmitter that could be localized by satellites from space – in a pioneering project of our institute in the 1980s. Those first devices only had an accuracy of several hundred kilometers and often broke after only a few months. They have now been replaced by modern communication technology which enable us to locate the bird every few minutes with an accuracy of a few meters, and given good conditions, can last for several years. (A stork can live to be 30 years old.)
These trip recorders weigh almost 60g including harness and can easily be carried like a little backpack by storks of 3.5 to 4kg body mass. They collect GPS coordinates every 5 minutes, store them and tell us in programmable time intervals through a mobile phone network where they are. Thus, the tags report the stork’s current location daily, whether other storks with transmitters are around, which weather conditions it awaits before it crosses the sea at Gibraltar, whether it prefers to stay at Spanish landfills rather than continue to Africa, whether an increased appearance of White Storks indicates outbreaks of migrating locusts and much more.
So far, we have been able to electronically follow several hundred storks from 12 populations between Spain and Uzbekistan. Tracks of the storks are available for open access at Movebank and in the Animal Tracker app. They not only enable scientists to travel with the impressive long distance migrants on their journey from our latitudes in late summer over several thousand kilometers until they show up in spring at their nests again – over a stork’s whole life!