Collective movement: individual repeatability in subgroup size and departure order for subgroup flight departures in Greylag geese

Abstract

Abstract: The study of collective movement is at the forefront of understanding emergent properties of groups, such as collective intelligence and collective memory. If individuals consistently occupy certain social roles or are members of certain social groups, this consistency could affect the speed and accuracy, as well as the temporal properties, with which information spreads and/or is maintained within a group. In this study, we ask the fundamental question of whether subgroup membership and relative departure time are individually repeatable across days and years in an individually color-banded population of greylag geese (Anser anser). The system is well-suited for the proposed questions because we have ‘near-complete’ life history information for each flock member. We recorded the identity of a focal goose and its subgroup size at time of departure, as well as the relative order in which subgroups departed in flight over a year. Results showed significant individual consistency in subgroup size and relative departure time within and between years. Our results underscore the role of individual identity in certain features of subgroup movement in this study system, with implications for modelling the spread of new information when subgroup membership is conserved. Finally, consistent individual differences carry important implications for the management and protection of wildlife in the face of rapidly changing environmental conditions. Significance statement: In addition to simple rules of thumb, collective movement can also be influenced by characteristics of individual members of a group. In this study, we take advantage of a long-term color-banded population of greylag geese for which we have nearly complete life history data over 70 years. We asked if there is consistency in subgroup size and relative departure time for individual geese that participate in collective subgroup movement events in the wild. In both cases, we found strong repeatability across days and years for subgroup size and departure time for individual geese. These findings will help to better understand the role of phenotypic heterogeneity and consistency for understanding emergent properties of collective movement.