From local collective behavior to global migrations
What role do social dynamics play in guiding collective migrations? Identifying such dynamics requires following individual animals across long migratory distances. Our latest paper in Science [Flack & Nagy et al. (2018)] used GPS tags to follow individual juvenile white storks on their southern migration. Birds generally fell into two categories: leaders and followers. Leaders sought out areas of thermal uplift, flapped less in transit, and flew farther. Followers followed leaders into thermals but had different trajectories, exhibited greater flapping effort, and flew shorter total distances.
Coordination and synchronization during thermalling.
Our recent study (Nagy et al. 2018) explored how migrating birds interact with each other in thermals. To do so, we analysed high-resolution GPS trajectories of individuals from a flock of white storks during circling events. An analysis of 1,414,226 locations revealed that when storks are circling together in a thermal updraft, different flock members experience different areas of the thermals—thus allowing the group to better explore the complex structure through the process of collective sensing. Further, the storks are coordinating their motion to circle together and forming dynamically changing subgroups—allowing them to use the thermals most efficiently.
Wind estimation based on thermal soaring of birds. Our new paper in Ecology and Evolution (Weinzierl et al. 2016) introduces a systematic approach to evaluate wind speed from high-frequency GPS recordings of birds during thermalling flight. To evaluate the accuracy of our approach, we use a large dataset of GPS-tagged migrating storks. We validate our results by comparing its wind estimates with the mid-resolution weather reanalysis data from ECMWF, and by examining independent wind estimates from pairs of birds that are flying in close proximity. Our approach provides accurate and unbiased observations of wind speed and additionally detailed information on vertical winds and uplift structure.
Costs of migration: Comparison of white stork populations. In our paper in Science Advances (Flack et al. 2016), we uncover the large extent of variation in the lifetime migratory decisions of young white storks originating from eight populations. Not only did juvenile storks differ in their geographically distinct wintering locations, their diverse migration patterns also affected the amount of energy individuals invested for locomotion during the first months of their life. Overwintering in areas with higher
human population reduced the stork’s overall energy expenditure because of shorter daily foraging trips, closer wintering grounds, or a complete suppression of migration.
Using the white stork as a model system, I would like to bring collective behaviour research into the field.
In the near future, my goal is to move my research beyond in situ observations. I would like to improve our understanding on group dynamics and their ecological and evolutionary consequences by performing carefully designed experiments with wild animals.
What are leaders made of? Previous navigational experience contributes to the establishment of leader-follower relationships. The larger the difference in experience between two co-navigating pigeons, the higher the likelihood the more experienced bird will emerge as leader. The results were published in Animal Behaviour (Flack et al. 2012).
Robustness of leadership hierarchies. In this study (Flack et al. 2013) we found that ladership hierarchies in pigeon flocks appear resistant to changes in the navigational knowledge of a subset of their members, at least when these changes are relatively small in magnitude. The stability of the hierarchical structure might be beneficial during decision-making.
Modelling group navigation. We studied the impact of social relationships on group navigation by introducing social network structures into a model of collective motion. Our results show that groups consisting of equally informed individuals achieve the highest level of accuracy when they are hierarchically organized with the minimum number of preferred connections per individual. We also observe that the navigational accuracy of a group will depend strongly on detailed aspects of its social organization. These results were published in the Royal Society journal Interface (Flack et al. 2014).
Pairs of pigeons act as behavioural unit. In this paper in JEB (Flack et al. 2013) we showed that shared homing experience through repeated joint flights can allow two pigeons to develop into a “behavioural unit”. They form spatial sub-groups when flying with less familiar birds, and perform a similar transition between compromise- and leadershipdominated flights as single birds, although they are more likely to accept compromise routes. Such previous association histories between birds can thus affect collective decision-making in larger flocks.
Learning multiple homing routes. We observed that pigeons successfully develop and apply memories of the three different release sites, irrespective of the sequence of the training releases, and that learning several routes in parallel did not impair their capacity to quickly improve their homing efficiency over multiple releases. These findings can be read in Biology Letters (Flack et al. 2014).
During my undergraduate studies, I investigated the famous honeybee dance. My diploma thesis (Spatial knowledge and dance communication in honeybees) described that a honeybee’s response to the dance depends strongly upon its past navigational experience, and upon the spatial knowledge that is shared -or not shared- by its colony members.