The impact of beachface morphology on habitat suitability and breeding success of least terns and snowy plovers

Beach-nesting birds rely on predictable and stable beach conditions for successful breeding, yet the complex impacts of climatic events, wave action, and variation in beach morphology on nest site selection remain underexplored. This study examines how variations in beach morphology influence nest site selection and breeding success in two conservation-priority species —the least tern ( Sterna antillarum ) and the snowy plover ( Anarhynchus nivosus )—at Barra del Estero Beach, Baja California, Mexico. This beach, formed in the mid-to-late 1990s, likely due to alongshore sediment transport from neighboring beaches during energetic El Niño events, reveals that nest abundance correlates strongly with supratidal beach morphology. Both species exhibited significantly higher survival rates when nesting on berms compared to other areas. They presented highest nest densities on stable profiles featuring prominent berms extending approximately 40 m horizontally from the dune toe at elevations exceeding 2.8 m, 0.5 m above the highest spring high-tide level. While berms erode in winter due to high-energy waves, they serve as secure nesting platforms in summer, protecting nests from flooding. Thus, the ability of the beach to rebuild wide berms after the winter is crucial for nesting success. As sea-level rise and extreme weather events increasingly alter beach morphology, species with greater flexibility in nest site selection may more successfully identify higher-quality nesting sites. This research highlights the urgent need for comprehensive solutions to mitigate the growing risk of nest flooding in beach-nesting birds.