Population dynamics of breeding seabirds and diet of common breeding seabirds along the coast of Zhejiang Province, China.

Seabirds are a key indicator species of marine. Understanding their diet is of great significance for assessing coastal trophic structure and formulating conservation strategies. Based on integrated citizen science data from 2021 to 2025 and DNA metabarcoding technology, we analyzed population dynamics of breeding seabirds along the coast of Zhejiang Province, as well as the dietary characteristics of two common species, Larus crassirostris and Onychoprion anaethetus, during the breeding season. Citizen science data indicated that the coastal area of Zhejiang served as an important habitat for seabirds. A total of 95269 seabirds were recorded in the past five years. Zhoushan Archipelago had become the core breeding habitat for seabirds. High-throughput sequencing based on DNA metabarcoding of fecal samples from L. crassirostris and O. anaethetus identified a total of 36 fish species and 30 crustacean species. There was dietary differentiation between L. crassirostris and O. anaethetus. L. crassirostris mainly preyed on marine fish of Mugilidae and Engraulidae, while O. anaethetus prefer marine fish of Clupeidae and Mullidae. Both species fed on arthropods of Macrophthalmidae and Portunidae. In addition, L. crassirostris consumed the freshwater fish Micropterus salmoides, revealing dietary plasticity in the ecotone between marine and terrestrial habitats. Due to limited sampling of O. anaethetus, dietary diversity analysis was only performed on L. crassirostris from multiple breeding islands. The results showed that L. crassirostris from Wuzhushan Island of Zhoushan had the highest dietary composition diversity, without differences among islands. This study filled the gap in basic dietary data of common seabirds along the coast of Zhejiang Province and provides scientific support for the conservation of coastal biodiversity.