Three-dimensional trajectories and network analyses of group behaviour within chimney swift flocks during approaches to the roost.

Dennis J Evangelista,Tyson L Hedrick,Dylan D Ray,Sathish K Raja

doi:10.1098/rspb.2016.2602

Dennis J Evangelista, Tyson L Hedrick + Show 2 more

Open Access

https://doi.org/10.1098/rspb.2016.2602

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Abstract

Chimney swifts (Chaetura pelagica) are highly manoeuvrable birds notable for roosting overnight in chimneys, in groups of hundreds or thousands of birds, before and during their autumn migration. At dusk, birds gather in large numbers from surrounding areas near a roost site. The whole flock then employs an orderly, but dynamic, circling approach pattern before rapidly entering a small aperture en masse. We recorded the three-dimensional trajectories of ≈1 800 individual birds during a 30 min period encompassing flock formation, circling, and landing, and used these trajectories to test several hypotheses relating to flock or group behaviour. Specifically, we investigated whether the swifts use local interaction rules based on topological distance (e.g. the n nearest neighbours, regardless of their distance) rather than physical distance (e.g. neighbours within x m, regardless of number) to guide interactions, whether the chimney entry zone is more or less cooperative than the surrounding flock, and whether the characteristic subgroup size is constant or varies with flock density. We found that the swift flock is structured around local rules based on physical distance, that subgroup size increases with density, and that there exist regions of the flock that are less cooperative than others, in particular the chimney entry zone.

Highlights

The movement of groups of animals, especially the coordinated behaviour of birds in flocks, has excited observers and researchers for many years leading to studies from a variety of biological and mathematical perspectives
Some of the behavioural details of these underlying tasks may in turn affect how pairwise and higher-order local rules may govern flocking behaviour
We use long duration (%30 min), high temporal resolution (30 frames s21) three-dimensional tracks reconstructed from video recordings of a flock of %1 800 chimney swifts (Chaetura pelagica) circling and landing in a chimney at dusk, to probe underlying local interaction rules and variation in flock interactions and structure spatially and with time

Summary

Introduction

The movement of groups of animals, especially the coordinated behaviour of birds in flocks, has excited observers and researchers for many years leading to studies from a variety of biological and mathematical perspectives. We analysed slices in which a large number of birds were present in the recording volume, and the flock used a single elliptical approach pattern throughout, rather than during transient events such as a reversal in direction around the chimney, a split into two separate approach patterns, or merging of two patterns into one In each of these time slices, we quantified how the average bird metrics and the properties of the network varied spatially to address our hypothesis that the flock would become more competitive (i.e. less cooperative) near the chimney roost. We examined the distribution of entry times at the chimney and control volume to see if chimney entries were clumped or distributed in time compared to elsewhere in the flock

Results

Findings

Discussion