Greylag geese (Anser anser) initiate group flight based on the number and identities of callers

Abstract

Social animals need to maintain cohesion when they move, and movement decisions are influenced by social organization and ecological or individual needs. To maintain synchrony during collective movement, the use of signals, including vocal signals, is often necessary. Greylag geese are gregarious and vocal before and during flight. We observed instances of naturally occurring flight initiation attempts in a free-living population of greylag geese -n = 133 birds- to investigate the decision-making process in this species, including the role vocalisations play in flight decisions. We recorded the number of participants in flight-initiation attempts, their vocal behaviour -using two call types specific to flight initiation signalling: Locomotion and Pre-flight calls-, the outcome of those attempts, in addition to the resulting flying group size and composition. We found that the identities and number of flock members joining in the calling chorus predicted both the probability of successful flight and the resulting flying group size. The number of Locomotion callers was associated with an increased probability of success, as well as larger flying group size, including both family and non-family fliers. The number of Pre-flight callers was also associated with a higher probability of successful flight, but its effect on the increase in flying group size was marginal. Successful departures were more probable when the family members of the initiator responded vocally. Overall, our findings are consistent with a system where the decision to fly is made individually using two distinct vocal signals to estimate the number of voters and at least the identities of social partners and progeny. In species where temporal and spatial group cohesion varies both seasonally and diurnally, individuals priorities appear to be maintaining proximity to mates and family members, followed by maximizing the size of flight groups.