Hugh Trenchard, Victoria, British Columbia, Canada

Abstract: American coot (Fulica americana) flocks exhibit water surface (two-dimensional) collective dynamics that oscillate between two primary phases: a disordered phase of low density and non-uniform coot body orientations; a synchronized phase characterized by high density, uniform body orientations and speed. For this small-scale study, data was obtained for flocks of 10 to ~250 members for these phases. Approximate durations of phase occurrences were recorded for different flock sizes and for both relatively calm and severe weather conditions. Results indicate that for timed durations of up to ~ 2 hours, small flocks (10 coots) exhibit a comparatively high disordered/synchronized phase ratio (PR) in relatively warm and well-sheltered conditions (substantially > 1); large flocks (~100 or more) generally exhibit a PR near 1, while large flocks spend comparatively more time in a disordered phase in relatively calm conditions (PR somewhat > 1), and spend more time in a synchronized phase during severe conditions (PR < 1). Data suggests a correlation between flock size and PR; and weather conditions and PR, whereby coupling principles driving on-water collective behavior include energy-savings and thermoregu-lation. Secondary phases occur, including expanding circle and sequential short distance flight, near-single-file lines, convex and concave arcs, among others.

Keywords: American coot, collective self-organization, two-dimensional flocking behavior, phase-transition, synchronization