Flocking and flushing behavior in roosting gull (Laridae) flocks on San Juan Island

Abstract

Roosting behavior is known to provide a variety of benefits to flocking birds; however, the behavior of roosting gulls has received limited study. In particular, the impact of human disturbance on these roosting flocks, especially as human activity on beaches increases, is the subject of almost no recent research. To address this knowledge gap, we collected abundance, behavioral, and disturbance data at two sites on San Juan Island. We found some evidence for behavior variation based on time of day, position in the flock, and age, as well as strong evidence for behavior variation based on site. Specifically, gulls at South Beach, a site with more human disturbance, spent more time alert and moving; gulls at Argyle Lagoon, a site with less human disturbance, spent more time resting and preening. Most flushing events we observed involved dogs (70.2%) or only humans (19.1%); in general, flushing frequency was correlated with human and dog abundance. After flushing, gulls were either displaced down the beach or into the water; repeated disturbance eventually resulted in all gulls being displaced to the water. Our findings help characterize the behavior of gulls in roosting flocks and demonstrate that these flocks are frequently impacted by disturbance, which may result in frequent energetic or opportunity costs. Long-term research is necessary to determine whether these costs impact gulls at the population level and across longer time scales. We recommend further research in this field to continue expanding our understanding of gull flocking behavior and the impact of human disturbance on coastal wildlife.