Andrea Flack

What are leaders made of? The role of individual experience in determining leader–follower relations in homing pigeons

Negotiating joint routes during group travel is one of the challenges faced by collectively moving animals, on spatial scales ranging from daily foraging trips to long-distance migrations. Homing pigeons, Columba livia, provide a useful model system for studying the mechanisms of group decision making in the context of navigation, owing to the combination of their gregarious nature and the depth of our understanding of their individual orientational strategies. Previous work has shown that during paired flight, if two birds’ individually preferred routes are sufficiently different, one bird will emerge as leader whom the other follows. What determines the identity of a leader has important implications for the efficiency of a moving collective, since leaders with higher navigational certainty can increase the accuracy of the group. We examined factors contributing to the establishment of leadership/followership, focusing on the role of previous navigational experience. We tested, on a homing task, pairs of pigeons in which the two partners had relatively greater and lesser prior experience, generated through individual training. Analysis of the GPS-tracked routes taken by such pairs revealed a negative correlation between homing experience and the probability that a pigeon would follow a co-navigating partner. Thus, the larger the difference in experience between two partners, the higher the likelihood the more experienced bird would emerge as leader. Our results contribute to a better understanding of the mechanisms and potential payoffs of collective navigational decision making in species that travel in mixed-experience groups.

Costs of migratory decisions: A comparison across eight white stork populations

Researchers uncover a large variation in the lifetime migratory decisions of young white storks. Annual migratory movements can range from a few tens to thousands of kilometers, creating unique energetic requirements for each specific species and journey. Even within the same species, migration costs can vary largely because of flexible, opportunistic life history strategies. 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. Because migrants can change ecological processes in several distinct communities simultaneously, understanding their life history decisions helps not only to protect migratory species but also to conserve stable ecosystems.

Not just passengers: pigeons, Columba livia , can learn homing routes while flying with a more experienced conspecific

For animals that travel in groups, the directional choices of conspecifics are potentially a rich source of information for spatial learning. In this study, we investigate how the opportunity to follow a locally experienced demonstrator affects route learning by pigeons over repeated homing flights. This test of social influences on navigation takes advantage of the individually distinctive routes that pigeons establish when trained alone. We found that pigeons learn routes just as effectively while flying with a partner as control pigeons do while flying alone. However, rather than learning the exact route of the demonstrator, the paired routes shifted over repeated flights, which suggests that the birds with less local experience also took an active role in the navigational task. The efficiency of the original routes was a key factor in how far they shifted, with less efficient routes undergoing the greatest changes. In this context, inefficient routes are unlikely to be maintained through repeated rounds of social transmission, and instead more efficient routes are achieved because of the interaction between social learning and information pooling.

Modelling group navigation: transitive social structures improve navigational performance.

Collective navigation demands that group members reach consensus on which path to follow, a task that might become more challenging when the groups members have different social connections. Group decision-making mechanisms have been studied successfully in the past using individual-based modelling, although many of these studies have neglected the role of social connections between the groups interacting members. Nevertheless, empirical studies have demonstrated that individual recognition, previous shared experiences and inter-individual familiarity can influence the cohesion and the dynamics of the group as well as the relative spatial positions of specific individuals within it. Here, we use models of collective motion to study 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. More specifically, group navigation does not only depend on the underlying social relationships, but also on how much weight leading individuals put on following others. Also, we show that groups with certain social structures can compensate better for an increased level of navigational error. The results have broader implications for studies on collective navigation and motion because they show that only by considering a groups social system can we fully elucidate the dynamics and advantages of joint movements.

Collective animal navigation and migratory culture : from theoretical models to empirical evidence

Animals often travel in groups, and their navigational decisions can be influenced by social interactions. Both theory and empirical observations suggest that such collective navigation can result in individuals improving their ability to find their way and could be one of the key benefits of sociality for these species. Here, we provide an overview of the potential mechanisms underlying collective navigation, review the known, and supposed, empirical evidence for such behaviour and highlight interesting directions for future research. We further explore how both social and collective learning during group navigation could lead to the accumulation of knowledge at the population level, resulting in the emergence of migratory culture. This article is part of the theme issue ‘Collective movement ecology’.

Synchronization, coordination and collective sensing during thermalling flight of freely migrating white storks

Exploring how flocks of soaring migrants manage to achieve and maintain coordination while exploiting thermal updrafts is important for understanding how collective movements can enhance the sensing of the surrounding environment. Here we examined the structural organization of a group of circling white storks (Ciconia ciconia) throughout their migratory journey from Germany to Spain. We analysed individual high-resolution GPS trajectories of storks during circling events, and evaluated each birds flight behaviour in relation to its flock members. Within the flock, we identified subgroups that synchronize their movements and coordinate switches in their circling direction within thermals. These switches in direction can be initiated by any individual of the subgroup, irrespective of how advanced its relative vertical position is, and occur at specific horizontal locations within the thermal allowing the storks to remain within the thermal. Using the motion of all flock members, we were able to examine the dynamic variation of airflow within the thermals and to determine the specific environmental conditions surrounding the flock. With an increasing amount of high-resolution GPS tracking, we may soon be able to use these animals as distributed sensors providing us with a new means to obtain a detailed knowledge of our environment. This article is part of the theme issue ‘Collective movement ecology’.

Pairs of pigeons act as behavioural units during route learning and co-navigational leadership conflicts

SUMMARY In many species, group members obtain benefits from moving collectively, such as enhanced foraging efficiency or increased predator detection. In situations where the groups decision involves integrating individual preferences, group cohesion can lead to more accurate outcomes than solitary decisions. In homing pigeons, a classic model in avian orientation studies, individuals learn habitual routes home, but whether and how co-navigating birds acquire and share route-based information is unknown. Using miniature GPS loggers, we examined these questions by first training pairs (the smallest possible flocks) of pigeons together, and then releasing them with other pairs that had received separate pair-training. Our results show that, much like solitary individuals, pairs of birds are able to establish idiosyncratic routes that they recapitulate together faithfully. Also, when homing with other pairs they exhibit a transition from a compromise- to a leadership-like mechanism of conflict resolution as a function of the degree of disagreement (distance separating the two preferred routes) between the two pairs, although pairs tolerate a greater range of disagreements prior to the transition than do single birds. We conclude that through shared experiences during past decision-making, pairs of individuals can become units so closely coordinated that their behaviour resembles that of single birds. This has implications for the behaviour of larger groups, within which certain individuals have closer social affiliations or share a history of previous associations.

From local collective behavior to global migratory patterns in white storks

GPS tagging reveals leaders and followers among migrating white storks. Follow the leader What role do social dynamics play in guiding collective migrations? Identifying such dynamics requires following individual animals across long migratory distances. Flack et al. used GPS tags to follow individual juvenile white storks on their southern migration (see the Perspective by Nevitt). 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. Science, this issue p. 911; see also p. 852 Soaring migrant birds exploit columns of rising air (thermals) to cover large distances with minimal energy. Using social information while locating thermals may benefit such birds, but examining collective movements in wild migrants has been a major challenge for researchers. We investigated the group movements of a flock of 27 naturally migrating juvenile white storks by using high-resolution GPS and accelerometers. Analyzing individual and group movements on multiple scales revealed that a small number of leaders navigated to and explored thermals, whereas followers benefited from their movements. Despite this benefit, followers often left thermals earlier and at lower height, and consequently they had to flap considerably more. Followers also migrated less far annually than did leaders. We provide insights into the interactions between freely flying social migrants and the costs and benefits of collective movement in natural populations.

Resolution of navigational conflict in king penguin chicks

Conflicts may arise within a moving animal group if its members have different preferred destinations. Many theoretical models suggest that in maintaining group cohesion conflicting preferences can have an overwhelming influence on decision making. However, empirical studies, especially on wild animals, remain limited. Here, we introduce a new study system for investigating collective decision making: king penguins, Aptenodytes patagonicus. Their gregarious lifestyle, the colonys organization into subgroups and group travel make king penguins especially interesting for studying collective movements. Chicks spend their first year of life in groups with other chicks (creches), and if displaced will return to their creche. We examined how different levels of navigational conflict affect such homing, by comparing the performance of pairs of chicks from the same creche with pairs from different creches. The majority of chicks in both treatments travelled at least part of the journey together; when doing so they were more efficient and faster than individuals travelling alone. Chicks took turns in leading and following. Chicks with a common destination (same-creche pairs) were more precise at homing and less likely to split up than those with a conflict over preferred destinations (different-creche pairs). Our results support some, but not all, predictions derived from theoretical models.

Learning multiple routes in homing pigeons

The aerial lifestyle of central-place foraging birds allows wide-ranging movements, raising fundamental questions about their remarkable navigation and memory systems. For example, we know that pigeons (Columba livia), long-standing models for avian navigation, rely on individually distinct routes when homing from familiar sites. But it remains unknown how they cope with the task of learning several routes in parallel. Here, we examined how learning multiple routes influences homing in pigeons. We subjected groups of pigeons to different training protocols, defined by the sequence in which they were repeatedly released from three different sites, either sequentially, in rotation or randomly. We observed that pigeons from all groups successfully developed and applied memories of the different release sites (RSs), irrespective of the training protocol, and that learning several routes in parallel did not impair their capacity to quickly improve their homing efficiency over multiple releases. Our data also indicated that they coped with increasing RS uncertainty by adjusting both their initial behaviour upon release and subsequent homing efficiency. The results of our study broaden our understanding of avian route following and open new possibilities for studying learning and memory in free-flying animals.