Lead or follow and quit earlier – Stork migration strategies according to latest research

Hungarian and German researchers have found that migrating storks belong to two “personality types” – leaders explore air currents, while followers tend to follow their trails. The choice of strategy is also a significant marker of the final stage of the journey. Máté Nagy and his colleagues published their results in the latest issue of Science.

June 4, 2018

Every year welcomes news about the following of the routes of migrating storks; thanks to Báró, Hajdú and other birds equipped with signallers, we know much more about their routes than a few years ago. Máté Nagy, a member of the MTA-ELTE Statistical and Biological Physics Research Group, and his colleagues from the Max Planck Institute for Ornithology and the University of Konstanz, Germany have elevated the research of stork migration to a new level.

White stork (Ciconia ciconia) Source: pixabay.com

Small, sixty-gram tracking devices were attached to more than sixty freshly hatched storks, which sent data about the exact position of the storks at short intervals, and also provided information about their manner of movement. These tiny instruments – which can also be found in the simplest smartphones – can detect whether a stork is gliding, changing its direction fast or is moving its wings. The hatchlings were from a small stork population nesting near Radolfzell, Germany, and they often belonged to the same brood; thus, researchers expected them to form groups during migration. The behaviour of such groups had never been documented and analysed in such depth before.

Leaders and followers

Everyone who watches the flight of birds knows that birds use two methods to fly: they either flap their wings or they glide. Flapping requires considerable muscle movement and energy. Gliding, however, is much “cheaper” in this respect, although the distance that can be covered by gliding is limited, as the birds are constantly sinking. Thermals (columns of rising air) are exploited by birds so they can rise higher with minimal effort. By gliding from thermal to thermal, several hundreds of kilometers may be covered without one flap of a wing, as proved by the distance records of paragliders.

The two “personality types” of storks gliding in thermals. Blue: leader’s route; red: follower’s route.
Source: Youtube/Max Planck Institute for Ornithology/R. Bastien and M. Nagy

As the data collected by the research group headed by Professor Martin Wikelski show, storks can be grouped into two “personality types”: leaders and followers. Leaders are skilled thermal detectors; they find rising columns of air easily, which lift them higher and higher. Consequently, these birds follow a somewhat irregular route, as they constantly try to find new opportunities to rise higher. They flap their wings quite rarely, as thermals regularly raise them. When they reach the maximal height of the thermal, they glide over to the next one.

The strategy of followers is quite different from this. They do not experiment, but wait for leaders to find strong thermals and follow them. As a consequence, their routes are much more regular. However, to be able to keep up with leaders, they must leave the thermal sooner and flap their wings energetically to help them reach the next thermal.

The choice between these two strategies not only determines daily flying patterns, but also the destination of migrations. Researchers have found that leaders tend to head for West Africa, while followers generally stop in Spain, hardly covering half the distance of leaders, who are more experienced in detecting thermals. The correlation is so strong that it is enough to examine a few minutes of the migratory flight of a certain bird, allowing the viewer to determine whether it is heading to Africa or will be content with the Iberian Peninsula.

Routes of storks in the first four weeks of their migration. The warmer the colour, the higher the number of wing flaps. Source: Youtube/Max Planck Ornitológiai Intézet/R. Bastien and M. Nagy

Mobiles for storks with roaming tariffs

While it seems storks are not at all disturbed by the weight of the tracking device, the size of the collected database is really a burden – at least for the budget of the project. The information collected reaches the researchers’ database through the public mobile network. In the case of several dozens of storks migrating for several weeks, the cost of West African roaming tariffs is extremely high. Additionally, the constant use of GPS and regular texting by the tiny unit operated by a photovoltaic cell require a lot of energy, as all smartphone users know. Andrea Flack, a researcher at the Max Planck Institute for Ornithology, followed the storks by car as far as Spain, and downloaded the data to their own base station. When the storks reached Africa, the researchers modified the frequency of sampling to reduce costs and energy demand.

Fortunately, as a result of the researchers’ several decades of hard work, a new solution is in sight: from August 2018, the tracking devices can send data through the International Space Station (ISS) thanks to the Icarus Initiative, overcoming the limitations of networks on Earth.

A Youtube video abstract of the results with English subtitles

The English summary of the Max Planck Institute for Ornithology can be read here.

The article published in Science is available here.

For further information, contact Máté Nagy (Max Planck Institute for Ornithology, MTA-ELTE Statistical and Biological Physics Research Group) at nagymate [at] hal [dot] elte [dot] hu.