Structural characteristics of important daily movement corridors for waterbirds in urban areas: A case study on Black-headed Gulls

Landscape management facilitating animal movement is essential for sustaining urban wildlife populations and ecosystem services. Although linear vegetation corridors effectively conserve terrestrial animal movements, it remains unclear which landscape elements serve as pathways for aquatic organisms. Urban waterbirds frequently utilize rivers, but the specific characteristics that render certain river segments crucial as movement corridors have yet to be elucidated. This study aims to identify the characteristics of river segments functioning as critical corridors based on waterbird movement strategies. We tracked the movement behaviors of the Black-headed Gull (Chroicocephalus ridibundus), a river-dependent species in daily movements, and identified key movement corridors in highly urbanized Tokyo, Japan. Using GPS tracking data from gulls, we evaluated how river sinuosity and surrounding feature heights influenced tendencies to follow rivers across various temporal scales and perceptual ranges. Furthermore, we predicted and mapped these tendencies across individual river segments in central Tokyo. Gulls tended to fly along river segments with tall features during instantaneous to mid-term decision-making. In contrast, they utilized straight channels for long-term decisions. Although results varied slightly depending on the spatial scale they can perceive, this overarching trend remained consistent. Furthermore, our model predicted that the lower reaches of the Sumida River serve as critical movement corridors. This predictive tendency was also highly robust across all time scales. While straightened river segments with high feature heights may provide unsuitable habitats for diverse taxa, we emphasize their ecological value and argue that they should be conserved as essential movement corridors for waterbirds.