FLOCKS IN FLUX: EMERGENT DYNAMICS OF A FRAGMENTING MIGRATORY ROOST NETWORK

Abstract

Large-scale animal aggregations function as complex systems where emergent structures,such as herds, schools, or roosts, govern the movement of populations across entire continents. Despite the importance of these system-level behaviors, observing them at a continental scale has remained a significant challenge. The detection of aerial insectivore roosts on weather radar poses an opportunity to study an aggregation network on such a scale. With decades of radar-derived Tree Swallow (Tachycineta bicolor) roost sizes and locations, coupled with roost switching rates, we constructed a 19-year long multiplex network of migratory roosts across the Atlantic Flyway and assessed how its scale, composition, and topology have responded to population changes. Our novel method revealed nonrandomly-delineated roost communities, nonlinear population-dependent characteristics, and an overall fragmentation and contraction of the network as populations have declined. These results are the first to reveal the structural dynamics of a single-species animal aggregation network on a continental scale, providing a new framework for assessing the stability of collective systems under environmental stress.