Individual-level personality influences social foraging and collective behaviour in wild birds

Abstract

There is increasing evidence that animal groups can maintain coordinated behaviour and make collective decisions based on simple interaction rules. Effective collective action may be further facilitated by individual variation within groups, particularly through leader-follower polymorphisms. Recent studies have suggested that individual-level personality traits influence the degree to which individuals use social information, are attracted to conspecifics, or act as leaders/followers. However, evidence is equivocal and largely limited to laboratory studies. We use an automated data-collection system to conduct an experiment testing the relationship between personality and collective decision-making in the wild. First, we report that foraging flocks of great tits (Parus major) show strikingly synchronous behaviour. A predictive model of collective decision-making replicates patterns well, suggesting simple interaction rules are sufficient to explain the observed social behaviour. Second, within groups, individuals with more reactive personalities behave more collectively, moving to within-flock areas of higher density. By contrast, proactive individuals tend to move to and feed at spatial periphery of flocks. Finally, comparing alternative simulations of flocking with empirical data, we demonstrate that variation in personality promotes within-patch movement while maintaining group cohesion. Our results illustrate the importance of incorporating individual variability in models of social behaviour.

Keywords: Parus major; behavioural type; collective decision-making; group foraging; leader–follower; social information.

Figure 1.

(a) Collective behaviour of focal individuals in the population, with birds arriving more often to high-density sites and less often to low-density sites than expected. The y-axis is observed probability of arriving, given the density of individuals at the feeder (see equation (2.1)). The red envelope represents maximal range of data from 1000 jack-knife randomizations with 40% of data removed. The blue line is the result of 20 000 simulated arrivals using model with social attraction parameter s = 1.93. Numbers represent observed arrivals for each density bin. (b) Relationship between-individual personality score (slow-explorer, SE to fast-explorer, FE) and social attraction s with 95% CI. (c) Mean personality score with respect to foraging location density from 10 000 simulated arrivals for each value of s. More SE (blue) individuals have a lower tendency to arrive at low densities and a high tendency to arrive at high densities, whereas more FE (red) individuals have less deviation from random feeder choice. Line = 0 represents expected arrivals in the patch based on random feeder choice, open circles are the observed flock-level data from (a), colours are mean personality in simulated data for tendency to arrive at feeders with a given density.

Figure 2.

Relationship between group behaviour and personality composition: representative simulations of continuous arrivals and departures in a patch with a fixed population size of six birds, also see the electronic supplementary material, movies (S4). Legend to the right of plots shows colour range indicating proportion of population on a feeder. (a) Groups of only fast-exploring (FE) phenotypes never aggregate on a single feeder. (b) Groups of only average personality phenotypes fix at a single feeder. (c) Groups with slow-exploring (SE) phenotypes behave similarly to groups in (b), fixing at a single feeder. (d) Groups consisting of individuals with variable personality scores shift from one feeder to another.

Figure 3.

Emergent group behaviour arising from within-group variation in individual personalities, comparing observed data against four sets of simulations. Individuals in simulations were either: all fixed at s-value for most proactive individuals (FE); all fixed at average personality phenotype (average); all fixed at s-value for most reactive individuals (SE); or randomly sampled from within distribution of personality scores (variable). Horizontal bars are 95% range (simulated data) and 95% CI from 20 replicated habitat patches (empirical data). (a) Diversity of feeder use within patches in each simulation type, defined as how evenly all four feeders were used [60]. (b) Proportion of individuals in largest subgroup, representing group cohesion. Results highlight importance of intragroup variation in social information use, as also shown by empirical data.