Consistent individual differences in the social phenotypes of wild great tits, Parus major

Abstract

Despite growing interest in animal social networks, surprisingly little is known about whether individuals are consistent in their social network characteristics. Networks are rarely repeatedly sampled; yet an assumption of individual consistency in social behaviour is often made when drawing conclusions about the consequences of social processes and structure. A characterization of such social phenotypes is therefore vital to understanding the significance of social network structure for individual fitness outcomes, and for understanding the evolution and ecology of individual variation in social behaviour more broadly. Here, we measured foraging associations over three winters in a large PIT-tagged population of great tits, and used a range of social network metrics to quantify individual variation in social behaviour. We then examined repeatability in social behaviour over both short (week to week) and long (year to year) timescales, and investigated variation in repeatability across age and sex classes. Social behaviours were significantly repeatable across all timescales, with the highest repeatability observed in group size choice and unweighted degree, a measure of gregariousness. By conducting randomizations to control for the spatial and temporal distribution of individuals, we further show that differences in social phenotypes were not solely explained by within-population variation in local densities, but also reflected fine-scale variation in social decision making. Our results provide rare evidence of stable social phenotypes in a wild population of animals. Such stable social phenotypes can be targets of selection and may have important fitness consequences, both for individuals and for their social-foraging associates.

Keywords: Parus major; animal personality; repeatability; social behaviour; socixal network analysis.

Figure A1

The average number of locations per year where individuals of the different age and sex classes were observed foraging. Lines show SEs and data are used from three winter seasons. Numbers show the number of unique individuals recorded in each class.

Figure A2

The repeatability measures for the different sex classes, showing the 95% confidence interval for each measure. Nonoverlapping confidence intervals at the 95% level are marked with an asterisk. Group size and degree measures do not overlap at the 84% confidence interval (nonoverlapping 84% confidence intervals are equivalent to a z-test at the 0.05 level). All 41 repeated social networks across 3 years were used to calculate repeatability estimates, additionally controlling for year (1–3). Blue nodes are males; red nodes are females.

Figure A3

Simplex plots of repeatabilities for 210 individuals observed over three winter seasons of social network data collection. Each point represents one individual, and a perfect correlation between all three seasons is the central intercept of axes. Data are scaled between 0 and 1 within each season. Five winter-long measures of social behaviour are shown: (a) average group size, (b) unweighted degree, (c) betweenness centrality, (d) association strength and (e) clustering coefficient.

Figure 1

(a) Map of the study site showing the location of 65 feeding stations, each approximately 250 m apart and opening to scan for PIT-tagged great tits for 26–28 days of data collection over each of three winters. Smaller points on the map show the 1018 artificial nestboxes installed in the woodland. (b) An example of a social network constructed using this information on spatiotemporal foraging behaviour; the network is shown for the entire 2013–2014 winter period. Each node is one of 816 individuals and links between nodes are scaled between 0 (never observed in the same foraging flock) to 1 (always observed in the same foraging flock). Node size is scaled by unweighted degree (1–226).

Figure 2

Repeatabilities and 95% confidence intervals for average group size and four social network metrics: degree, betweenness, association strength and clustering coefficient (CC; between years only for the latter). Results are shown for three winter data collection periods (within-season repeatability): (a) 2011–2012 winter; (b) 2012–2013 winter; (c) 2013–2014 winter. (d) Results compared across 3 years (between-year repeatability). Estimates whose confidence intervals do not cross 0 (y-axis) are significantly repeatable at the α = 0.05 level. Horizontal red lines show 95% range of the repeatability estimates calculated from 1000 data randomizations controlling for spatial location.

Figure 3

Repeatabilities for 210 individuals observed over three winter seasons of social network data collection. Scores are scaled between 0 and 1 for all individuals within each season. Points are individuals and grey lines connect their scores in each year. Red lines show the average score for all individuals in each year. Five winter-long measures of social behaviour are shown: (a) average group size (R = 0.51), (b) degree (R = 0.55), (c) betweenness centrality (R = 0.33), (d) strength (R = 0.57) and (e) clustering coefficient (R = 0.43).