The Impact of Beachface Morphology on Habitat Suitability and Breeding Success of Least Terns and Snowy Plovers

Abstract Beach-nesting birds rely on predictable conditions for successful breeding, yet the effects of climatic events and beach morphology on nest placement remain underexplored. We examine how beach morphology influences nest placement and breeding success of least terns ( Sterna antillarum ) and snowy plovers ( Anarhynchus nivosus ) at Barra del Estero de Punta Banda, Baja California. Nest abundance along the northern beach is strongly linked to supratidal morphology. From 2018 to 2022, we monitored 411 least tern and 84 snowy plover nests. Of 162 nests with elevation data, 36% were successful and 11% failed due to flooding. We then examined nest placement effects, finding that across both species, nests located on berms had significantly higher survival (76%; 95% CI: 0.73–0.78) than those off berms (24%; 95% CI: 0–0.47). Furthermore, the highest nest densities were observed on dynamic profiles with prominent berms persisting across consecutive years. These berms extended approximately 40 m from the dune toe at elevations exceeding 2.8 m—approximately 0.5 m above the highest spring high-tide level. Although berms naturally erode in winter due to high-energy waves, they provide secure nesting platforms in summer that protect nests from flooding. Thus, the ability of the beach to rebuild wide berms after winter is crucial for nest success. As sea level rise and extreme weather alter beach morphology, species with greater flexibility in nest site selection may better find higher-quality nesting sites. Our research highlights the need for comprehensive solutions to mitigate the growing risk of nest flooding for beach-nesting birds.