Every autumn, around 2 billion birds fly south to reach warmer places. Some of the more noticeable members of this aerial exodus are storks—one of the largest species of land migrants on the planet.
Storks are iconic birds. They are also conspicuous—with more than one hundred thousand individuals migrating out of Europe every year. And after 100 years of research, it has become one of our most familiar long-distance migratory species.
But, despite never travelling alone, most of what we know about their migration comes from exploring the journeys of single birds. Their sociality during migration is still shrouded by big questions, such as: why do they travel in flocks of enormous size? Are they using information from their flock mates to find the most efficient routes, or the best wintering locations?
Now, with the help of new and sophisticated technology, scientists from the Max Planck Institute for Ornithology and University Konstanz have begun to shed light on this mystery.
“We attached harnesses containing GPS devices and Accelerometers to over 60 white storks, 27 of which migrated together as a flock,” says Dr Andrea Flack, a post-doctoral researcher from the Max Planck Institute for Ornithology (MPIO), and co-first author on the study.
These devices were key: they recorded incredibly high resolution tracks. By transmitting 1 data point every second, they allowed the scientists to glimpse for the first time how birds are interacting with each other during flight.
"During the first 1000 km we followed the birds, and directly downloaded these detailed movement tracks," says Dr Flack. "After that, for the entire lifetime of the storks, the devices were powered by solar panels, and sent us coarse location data using mobile phone networks.”
By studying the collective movement of a wild migratory species, the team have discovered a previously unknown link between social behaviour and global migration patterns.
“We see that birds are using social information from their peers, and that it is possible to predict global stork migrations based on local collective behavior,” says Dr Flack.
Leaders and followers
It has long been known that storks fly thousands of kilometres to reach their wintering locations. In order to complete their epic journey, they rely on updrafts of air—known as thermals—that they must locate and navigate to. Once they reach a thermal it is also a challenge to find the best region to circle in, as the air flow is dynamical and drifts with the wind, which could have different speeds and directions and altitudes. Flapping uses up precious energy, but soaring on thermals is a way of traveling while keeping energy costs down.
“The acceleration data that we collected from the tracking units allowed us to distinguish birds that fly by gliding between thermals, from those that mostly rely on flapping,” says Dr Máté Nagy, a post-doctoral researcher from the MPIO Department of Collective Behaviour who, as co-first author on the study, led the analyses of tracking data .
This led to a major discovery of the study: migrating storks choose between two kinds of complex strategies—leading and following. Birds at the front locate distant thermals and navigate towards them.
“Once inside the thermal, it is also these leaders who assess the strength of the uplift. They search for the best regions by changing their flight path— constantly flying in and out of the thermal lift,” says Dr Nagy. “On the other hand, storks flying behind are able to benefit from the leader’s costly exploration. By observing the leader’s movements, followers can selectively fly in the regions with the most updraft. However, when travelling between thermals, followers must flap considerably more, in order to keep up with the group.”
From local to global
But the consequences of these behavioural strategies extend far beyond the confines of the thermals. The results of the longer term tracking show that, ultimately, followers migrated shorter distances than the leaders did. It was the followers who ended their journey in Spain, while the most of the leaders travelled to Africa - some of them even all the way through the Sahara desert.
“By studying the collective movement of a wild migratory species, we have discovered a previously unknown link between social behaviour and global migration patterns,” says Dr Flack. “We see that migrating birds are using information from their peers, and that the social interplay between birds just a few metres apart can reverberate right up to the global level.”
For the scientific team led by Professor Martin Wikelski, the journey doesn’t end here. They want to find out if these different behavioural strategies used during migration have consequences for storks in the two things that matter mostly in their life: reproduction and mortality.
Background information on the migration of storks:
The natural habitat of the white stork (Ciconia ciconia) during the summer months stretches from the Iberian Peninsula to Central and Eastern Europe all the way to the Ural Mountains. There are also populations in Turkey, the Caucasus and in Central Asia.
The so-called eastern white storks in Europe fly to Africa in the fall by crossing the Bosporus, the Jordan Valley and the Sinai Peninsula. Their wintering grounds are located in East or South Africa. They require one to one and a half months to complete the 10,000 kilometre trip.
The western storks fly over the Mediterranean near Gibraltar and spend the winter in West Africa between Senegal and Lake Chad. These birds return to their breeding grounds in the spring before the eastern storks do. The approximate boundary separating the eastern and western stork runs roughly from the Elbe to Upper Bavaria. Only a few birds select the middle route over Italy to Tunisia in North Africa.
From local collective behavior to global migratory patterns in white storks
Andrea Flack, Máté Nagy, Wolfgang Fiedler, Iain D. Couzin, Martin Wikelski
Science; 25 May 2018