Relative importance of tidal flats and artificial habitats for two spoonbill species and related interspecific differences

Artificial/seminatural environments, such as aquacultural ponds, saltpans, and croplands, have recently been acknowledged as important habitats for coastal waterbirds. Although coastal waterbirds tend to use artificial habitats around tidal flats as roosting sites during high-tide, it remains unclear whether the importance of surrounding habitats relative to tidal flats varies among landscape types, seasons, species, or tidal conditions. The Black-faced Spoonbill (Platalea minor) and Eurasian Spoonbill (P. leucorodia) are two closely related sympatric species in East Asia with narrow and wide distribution ranges and habitat requirements, respectively. We therefore expect that both species will use surrounding artificial habitats across seasons at high tides, but Black-faced Spoonbills will use them less frequently than Eurasian Spoonbills. Here, we address these hypotheses in the Imazu tidal flat and its surrounding environments in southern Japan. We investigated the habitat use and behavioral patterns of both species through route and behavioral surveys during the fall migration and wintering seasons in 2021. We found that both species used surrounding habitats including artificial ones more frequently than the tidal flat regardless of the tidal condition or season, but spoonbills used these habitats more frequently in winter than in autumn. We also found that Eurasian Spoonbills foraged in surrounding artificial habitats more frequently than Black-faced Spoonbills. These results not only demonstrate how coastal waterbirds exploit surrounding habitats relative to tidal flats but also suggest that the importance of surrounding habitats varies among species and seasons. Our study thus emphasizes that valuing and managing surrounding habitats in addition to tidal flats are key to conserving globally declining waterbirds.