Publications by category
Journal articles
Sankey DWE (2022). 'Selfish herders' finish last in mobile animal groups.
Proceedings of the Royal Society B: Biological Sciences,
289(1985).
Abstract:
'Selfish herders' finish last in mobile animal groups
Predation is a powerful selective pressure and probably a driver of why many animal species live in groups. One key explanation for the evolution of sociality is the 'selfish herd' model, which describes how individuals who stay close to others effectively put neighbours between themselves and a predator to survive incoming attacks. This model is often illustrated with reference to herds of ungulates, schools of fish or flocks of birds. Yet in nature, when a predator strikes, herds are often found fleeing cohesively in the same direction, not jostling for position in the centre of the group. This paper highlights a critical assumption of the original model, namely that prey do not move in response to position of their predator. In this model, I relax this assumption and find that individuals who adopt 'selfish herd' behaviour are often more likely to be captured, because they end up at the back of a fleeing herd. By contrast, individuals that adopt a rule of 'neighbour to neighbour alignment' are able to avoid rearmost positions in a moving herd. Alignment is more successful than selfish herding across much of the parameter space, which may explain why highly aligned fleeing behaviour is commonly observed in nature.
Abstract.
Papadopoulou M, Hildenbrandt H, Sankey DWE, Portugal SJ, Hemelrijk CK (2022). Emergence of splits and collective turns in pigeon flocks under predation.
Royal Society Open Science,
9(2).
Abstract:
Emergence of splits and collective turns in pigeon flocks under predation
Complex patterns of collective behaviour may emerge through self-organization, from local interactions among individuals in a group. To understand what behavioural rules underlie these patterns, computational models are often necessary. These rules have not yet been systematically studied for bird flocks under predation. Here, we study airborne flocks of homing pigeons attacked by a robotic falcon, combining empirical data with a species-specific computational model of collective escape. By analysing GPS trajectories of flocking individuals, we identify two new patterns of collective escape: early splits and collective turns, occurring even at large distances from the predator. To examine their formation, we extend an agentbased model of pigeons with a discrete escape manoeuvre by a single initiator, namely a sudden turn interrupting the continuous coordinated motion of the group. Both splits and collective turns emerge from this rule. Their relative frequency depends on the angular velocity and position of the initiator in the flock: sharp turns by individuals at the periphery lead to more splits than collective turns. We confirm this association in the empirical data. Our study highlights the importance of discrete and uncoordinated manoeuvres in the collective escape of bird flocks and advocates the systematic study of their patterns across species.
Abstract.
Sankey DWE, Hunt KL, Croft DP, Franks DW, Green PA, Thompson FJ, Johnstone RA, Cant MA (2022). Leaders of war: Modelling the evolution of conflict among heterogeneous groups.
Philosophical Transactions of the Royal Society B: Biological Sciences,
377(1851).
Abstract:
Leaders of war: Modelling the evolution of conflict among heterogeneous groups
War, in human and animal societies, can be extremely costly but can also offer significant benefits to the victorious group. We might expect groups to go into battle when the potential benefits of victory (V) outweigh the costs of escalated conflict (C); however, V and C are unlikely to be distributed evenly in heterogeneous groups. For example, some leaders who make the decision to go to war may monopolize the benefits at little cost to themselves ( exploitative leaders). By contrast, other leaders may willingly pay increased costs, above and beyond their share of V ( heroic leaders). We investigated conflict initiation and conflict participation in an ecological model where single-leader multiple-follower groups came into conflict over natural resources. We found that small group size, low migration rate and frequent interaction between groups increased intergroup competition and the evolution of exploitative leadership, while converse patterns favoured increased intragroup competition and the emergence of heroic leaders. We also found evidence of an alternative leader/follower shared effort outcome. Parameters that favoured high contributing heroic leaders, and low contributing followers, facilitated transitions to more peaceful outcomes. We outline and discuss the key testable predictions of our model for empiricists studying intergroup conflict in humans and animals.
Abstract.
Sankey DWE, Biro D, Ricketts RL, Shepard ELC, Portugal SJ (2022). Pigeon leadership hierarchies are not dependent on environmental contexts or individual phenotypes.
Behavioural Processes,
198Abstract:
Pigeon leadership hierarchies are not dependent on environmental contexts or individual phenotypes
Remaining cohesive on the move can be beneficial for animal groups. As such, animal groups have evolved coordination mechanisms such as leadership to resolve navigational conflicts of interest. Consistent “leaders” may have an intrinsic advantage over “followers” which compromise on their preferred route to retain cohesion, which highlights the question of the inter-individual variation (phenotype) that can predict leadership. Studies in both birds and fish have revealed that intrinsically faster individuals can lead movements, and leading movements propagate from the front edge of the flock/shoal. However, these experiments are generally conducted in relatively “familiar” environments, where the degree of compromise between the “leaders” and “followers” is low. We suggested that inter-individual differences in route efficiency, while not explanatory of leadership from familiar locations, may emerge as predictors of leadership from unfamiliar locations. We tested this prediction – and the potential impact of multiple other behavioral, morphological and “in-flight” phenotypes on leadership – using two groups of homing pigeons (Columba livia) (N = 16), a classic model species of leadership. We recorded N = 966 unique GPS trajectories from birds in (i) solo and familiar, and (ii) solo and unfamiliar contexts to measure solo speed and solo route efficiency; and (iii) group and familiar, and (iv) group and unfamiliar contexts to assess group leadership. Pigeon leadership hierarchies were similar across environmental context (i.e. familiarity). However, we found that no covariates could consistently predict leadership score in either context.
Abstract.
Papadopoulou M, Hildenbrandt HL, Sankey DWE, Portugal SL, Hemelrijk C (2022). Self-organization of collective escape in pigeon flocks.
PLOS COMPUTATIONAL BIOLOGY,
18(1).
Author URL.
Sankey DWE, Storms RF, Musters RJ, Russell TW, Hemelrijk CK, Portugal SJ (2021). Absence of “selfish herd” dynamics in bird flocks under threat.
Current Biology,
31(14), 3192-3198.e7.
Abstract:
Absence of “selfish herd” dynamics in bird flocks under threat
The “selfish herd” hypothesis1 provides a potential mechanism to explain a ubiquitous phenomenon in nature: that of non-kin aggregations. Individuals in selfish herds are thought to benefit by reducing their own risk at the expense of conspecifics by attracting toward their neighbors’ positions1,2 or central locations in the aggregation.3–5 Alternatively, increased alignment with their neighbors’ orientation could reduce the chance of predation through information sharing6–8 or collective escape.6 Using both small and large flocks of homing pigeons (Columba livia; n = 8–10 or n = 27–34 individuals) tagged with 5-Hz GPS loggers and a GPS-tagged, remote-controlled model peregrine falcon (Falco peregrinus), we tested whether individuals increase their use of attraction over alignment when under perceived threat. We conducted n = 27 flights in treatment conditions, chased by the robotic “predator,” and n = 16 flights in control conditions (not chased). Despite responding strongly to the RobotFalcon—by turning away from its flight direction—individuals in treatment flocks demonstrated no increased attraction compared with control flocks, and this result held across both flock sizes. We suggest that mutualistic alignment is more advantageous than selfish attraction in groups with a high coincidence of individual and collective interests (adaptive hypothesis). However, we also explore alternative explanations, such as high cognitive demand under threat and collision avoidance (mechanistic hypotheses). We conclude that selfish herd may not be an appropriate paradigm for understanding the function of highly synchronous collective motion, as observed in bird flocks and perhaps also fish shoals and highly aligned mammal aggregations, such as moving herds.
Abstract.
Sankey DWE, O'Bryan LR, Garnier S, Cowlishaw G, Hopkins P, Holton M, Fürtbauer I, King AJ (2021). Consensus of travel direction is achieved by simple copying, not voting, in free-ranging goats. Royal Society Open Science, 8(2).
Ricketts RL, Sankey DWE, Tidswell BP, Brown J, Deegan JF, Portugal SJ (2021). Overall dynamic body acceleration as an indicator of dominance in Homing Pigeons (Columba livia). Ibis, 164(2), 581-586.
Portugal SJ, Usherwood JR, White CR, Sankey DWE, Wilson AM (2020). Artificial mass loading disrupts stable social order in pigeon dominance hierarchies. Biology Letters, 16(8).
Sankey DWE, Shepard ELC, Biro D, Portugal SJ (2019). Speed consensus and the ‘Goldilocks principle’ in flocking birds (Columba livia). Animal Behaviour, 157, 105-119.
Sankey DWE, Portugal SJ (2019). When flocking is costly: reduced cluster-flock density over long-duration flight in pigeons. The Science of Nature, 106(7-8).
Publications by year
2022
Sankey DWE (2022). 'Selfish herders' finish last in mobile animal groups.
Proceedings of the Royal Society B: Biological Sciences,
289(1985).
Abstract:
'Selfish herders' finish last in mobile animal groups
Predation is a powerful selective pressure and probably a driver of why many animal species live in groups. One key explanation for the evolution of sociality is the 'selfish herd' model, which describes how individuals who stay close to others effectively put neighbours between themselves and a predator to survive incoming attacks. This model is often illustrated with reference to herds of ungulates, schools of fish or flocks of birds. Yet in nature, when a predator strikes, herds are often found fleeing cohesively in the same direction, not jostling for position in the centre of the group. This paper highlights a critical assumption of the original model, namely that prey do not move in response to position of their predator. In this model, I relax this assumption and find that individuals who adopt 'selfish herd' behaviour are often more likely to be captured, because they end up at the back of a fleeing herd. By contrast, individuals that adopt a rule of 'neighbour to neighbour alignment' are able to avoid rearmost positions in a moving herd. Alignment is more successful than selfish herding across much of the parameter space, which may explain why highly aligned fleeing behaviour is commonly observed in nature.
Abstract.
Papadopoulou M, Hildenbrandt H, Sankey DWE, Portugal SJ, Hemelrijk CK (2022). Emergence of splits and collective turns in pigeon flocks under predation.
Royal Society Open Science,
9(2).
Abstract:
Emergence of splits and collective turns in pigeon flocks under predation
Complex patterns of collective behaviour may emerge through self-organization, from local interactions among individuals in a group. To understand what behavioural rules underlie these patterns, computational models are often necessary. These rules have not yet been systematically studied for bird flocks under predation. Here, we study airborne flocks of homing pigeons attacked by a robotic falcon, combining empirical data with a species-specific computational model of collective escape. By analysing GPS trajectories of flocking individuals, we identify two new patterns of collective escape: early splits and collective turns, occurring even at large distances from the predator. To examine their formation, we extend an agentbased model of pigeons with a discrete escape manoeuvre by a single initiator, namely a sudden turn interrupting the continuous coordinated motion of the group. Both splits and collective turns emerge from this rule. Their relative frequency depends on the angular velocity and position of the initiator in the flock: sharp turns by individuals at the periphery lead to more splits than collective turns. We confirm this association in the empirical data. Our study highlights the importance of discrete and uncoordinated manoeuvres in the collective escape of bird flocks and advocates the systematic study of their patterns across species.
Abstract.
Sankey DWE, Portugal SJ (2022). FLOCK STASIS DRIVES FLYING SPEED IN PIGEONS, WHILE ARTIFICIAL MASS ADDITIONS DO NOT.
Sankey DWE, Hunt KL, Croft DP, Franks DW, Green PA, Thompson FJ, Johnstone RA, Cant MA (2022). Leaders of war: Modelling the evolution of conflict among heterogeneous groups.
Philosophical Transactions of the Royal Society B: Biological Sciences,
377(1851).
Abstract:
Leaders of war: Modelling the evolution of conflict among heterogeneous groups
War, in human and animal societies, can be extremely costly but can also offer significant benefits to the victorious group. We might expect groups to go into battle when the potential benefits of victory (V) outweigh the costs of escalated conflict (C); however, V and C are unlikely to be distributed evenly in heterogeneous groups. For example, some leaders who make the decision to go to war may monopolize the benefits at little cost to themselves ( exploitative leaders). By contrast, other leaders may willingly pay increased costs, above and beyond their share of V ( heroic leaders). We investigated conflict initiation and conflict participation in an ecological model where single-leader multiple-follower groups came into conflict over natural resources. We found that small group size, low migration rate and frequent interaction between groups increased intergroup competition and the evolution of exploitative leadership, while converse patterns favoured increased intragroup competition and the emergence of heroic leaders. We also found evidence of an alternative leader/follower shared effort outcome. Parameters that favoured high contributing heroic leaders, and low contributing followers, facilitated transitions to more peaceful outcomes. We outline and discuss the key testable predictions of our model for empiricists studying intergroup conflict in humans and animals.
Abstract.
Sankey DWE, Biro D, Ricketts RL, Shepard ELC, Portugal SJ (2022). Pigeon leadership hierarchies are not dependent on environmental contexts or individual phenotypes.
Behavioural Processes,
198Abstract:
Pigeon leadership hierarchies are not dependent on environmental contexts or individual phenotypes
Remaining cohesive on the move can be beneficial for animal groups. As such, animal groups have evolved coordination mechanisms such as leadership to resolve navigational conflicts of interest. Consistent “leaders” may have an intrinsic advantage over “followers” which compromise on their preferred route to retain cohesion, which highlights the question of the inter-individual variation (phenotype) that can predict leadership. Studies in both birds and fish have revealed that intrinsically faster individuals can lead movements, and leading movements propagate from the front edge of the flock/shoal. However, these experiments are generally conducted in relatively “familiar” environments, where the degree of compromise between the “leaders” and “followers” is low. We suggested that inter-individual differences in route efficiency, while not explanatory of leadership from familiar locations, may emerge as predictors of leadership from unfamiliar locations. We tested this prediction – and the potential impact of multiple other behavioral, morphological and “in-flight” phenotypes on leadership – using two groups of homing pigeons (Columba livia) (N = 16), a classic model species of leadership. We recorded N = 966 unique GPS trajectories from birds in (i) solo and familiar, and (ii) solo and unfamiliar contexts to measure solo speed and solo route efficiency; and (iii) group and familiar, and (iv) group and unfamiliar contexts to assess group leadership. Pigeon leadership hierarchies were similar across environmental context (i.e. familiarity). However, we found that no covariates could consistently predict leadership score in either context.
Abstract.
Papadopoulou M, Hildenbrandt HL, Sankey DWE, Portugal SL, Hemelrijk C (2022). Self-organization of collective escape in pigeon flocks.
PLOS COMPUTATIONAL BIOLOGY,
18(1).
Author URL.
2021
Sankey DWE, Storms RF, Musters RJ, Russell TW, Hemelrijk CK, Portugal SJ (2021). Absence of “selfish herd” dynamics in bird flocks under threat.
Current Biology,
31(14), 3192-3198.e7.
Abstract:
Absence of “selfish herd” dynamics in bird flocks under threat
The “selfish herd” hypothesis1 provides a potential mechanism to explain a ubiquitous phenomenon in nature: that of non-kin aggregations. Individuals in selfish herds are thought to benefit by reducing their own risk at the expense of conspecifics by attracting toward their neighbors’ positions1,2 or central locations in the aggregation.3–5 Alternatively, increased alignment with their neighbors’ orientation could reduce the chance of predation through information sharing6–8 or collective escape.6 Using both small and large flocks of homing pigeons (Columba livia; n = 8–10 or n = 27–34 individuals) tagged with 5-Hz GPS loggers and a GPS-tagged, remote-controlled model peregrine falcon (Falco peregrinus), we tested whether individuals increase their use of attraction over alignment when under perceived threat. We conducted n = 27 flights in treatment conditions, chased by the robotic “predator,” and n = 16 flights in control conditions (not chased). Despite responding strongly to the RobotFalcon—by turning away from its flight direction—individuals in treatment flocks demonstrated no increased attraction compared with control flocks, and this result held across both flock sizes. We suggest that mutualistic alignment is more advantageous than selfish attraction in groups with a high coincidence of individual and collective interests (adaptive hypothesis). However, we also explore alternative explanations, such as high cognitive demand under threat and collision avoidance (mechanistic hypotheses). We conclude that selfish herd may not be an appropriate paradigm for understanding the function of highly synchronous collective motion, as observed in bird flocks and perhaps also fish shoals and highly aligned mammal aggregations, such as moving herds.
Abstract.
Sankey DWE, O'Bryan LR, Garnier S, Cowlishaw G, Hopkins P, Holton M, Fürtbauer I, King AJ (2021). Consensus of travel direction is achieved by simple copying, not voting, in free-ranging goats. Royal Society Open Science, 8(2).
Ricketts RL, Sankey DWE, Tidswell BP, Brown J, Deegan JF, Portugal SJ (2021). Overall dynamic body acceleration as an indicator of dominance in Homing Pigeons (Columba livia). Ibis, 164(2), 581-586.
Papadopoulou M, Hildenbrandt H, Sankey DWE, Portugal SJ, Hemelrijk CK (2021). Self-organization of collective escape in pigeon flocks.
2020
Portugal SJ, Usherwood JR, White CR, Sankey DWE, Wilson AM (2020). Artificial mass loading disrupts stable social order in pigeon dominance hierarchies. Biology Letters, 16(8).
Ricketts RL, Sankey DW, Tidswell BP, Brown J, Deegan JF, Portugal SJ (2020). Overall dynamic body acceleration as an indicator of dominance in homing pigeons.
2019
Sankey DWE, Shepard ELC, Biro D, Portugal SJ (2019). Speed consensus and the ‘Goldilocks principle’ in flocking birds (Columba livia). Animal Behaviour, 157, 105-119.
Sankey DWE, Portugal SJ (2019). When flocking is costly: reduced cluster-flock density over long-duration flight in pigeons. The Science of Nature, 106(7-8).