The white stork is a well-known migratory bird, and it has been found that they choose to migrate together in groups. A recent study from the Max Planck Institute of Animal Behavior in Germany has discovered that while all storks migrate with others of their kind, young storks are more influenced by the presence of their peers than adult storks. These birds with long legs and large wings fly from Europe to Africa in the fall and back again in the spring, often in large flocks that are a sign of the changing seasons. The researchers have collected data on the migration patterns of storks throughout their lifetimes.Of the 158 storks, the study is the first to provide evidence of storks showing social preference during migration. According to the researchers, the storks selected routes that were heavily used by other storks. Interestingly, young storks adjusted their routes to social hot-spots more than adults did. “It’s exciting to see the first clues that storks are actually choosing to fly with others,” says Hester Brønnvik, a doctoral student at the Max Planck Institute of Animal Behavior and first author on the study. “But as they gain migration experience, they also gain the independence to ignore social influences.” The findings suggest that storks exhibit social behavior during migration, but as they mature, they become more independent in their decision-making.The initial findings have provided insight into a long-standing question regarding the migration patterns of storks. According to Brønnvik, the observation of storks flying in groups does not necessarily indicate a deliberate choice to migrate together. It is possible that storks select their routes based on other factors such as favorable winds or a specific destination, which may result in them congregating in the same areas. In order to determine the social preferences of migrating storks, scientists needed to devise a method of communicating with the birds and understanding the reasons behind their route selection. This led to a study of route selection.
The team utilized a 10-year data set that contained the precise GPS locations of 158 storks from southern Germany, tracked every hour throughout their lifetime. “We were able to observe the complete migrations of these storks every year, from a three-month-old juvenile on its first migration to a highly experienced nine-year old taking one of its final flights,” Brønnvik explains.
Following this, the researchers assembled a picture of the social landscape that these 158 storks would have encountered during their migrations. They gathered data from 400 storks to estimate the locations of flocks and generate a social landscape.
The landscape. “This does not definitively tell us if storks flew with others,” Brønnvik explains. “Instead, it gives us a probability, indicating whether a stork’s route would likely intersect with other storks.” They then utilized a statistical model to compare the routes that storks flew with locations that they did not fly but had access to. “The model essentially asked why they chose the route they did,” says Brønnvik.
Experienced storks have less access to other storks
The analysis revealed that all storks, regardless of age, chose routes that had less access to other storks.The density of storks influenced the route selection of younger storks, but this influence decreased as they got older and more experienced with migration. This means that as birds aged, they were more willing to choose routes with fewer storks if it meant better flying conditions.
The authors suggest that this change in strategy over a stork’s lifetime could be due to younger birds relying on information from their peers to survive, as they have not yet gained enough knowledge from previous flights. “Storks need to locate invisible thermals for flight support, or find pockets of food during stopovers,” explains senior author Andrea Flack, who leads the Collective Migration group at the Max.The Planck Institute of Animal Behavior has found that following others could help animals find essential resources faster. However, as birds gain experience, they can become independent from the flock and wean themselves off this social information. According to Flack, this independence could help them time their migrations to meet their own reproductive goals.
The study’s innovation lies in the fact that the decision-making of long-haul migrants has been observed on a large scale. Flack stated, “Ultimately, we want to know how the decisions of migrating storks are affected by those around them. Our study provides the first clue of just how important the collective is to those birds.”
The study found that experience reduces route selection for conspecifics by the collectively migrating white stork, according to the researchers Hester Brønnvik, Elham Nourani, Wolfgang Fiedler, and Andrea Flack.
Journal Reference:
- Hester Brønnvik, Elham Nourani, Wolfgang Fiedler, Andrea Flack. Experience reduces route selection for conspecifics by the collectively migrating white stork. Current Biology, 2024; DOI: 10.1016/j.cub.2024.03.052