Diet composition of the Eurasian oystercatcher ( <i>Haematopus ostralegus osculans</i> ) during breeding season on Yubu Island through high-throughput DNA sequencing

Background: Prey availability plays a critical role in conserving endangered bird species, influencing their habitat use, reproductive success, and survival. Eurasian Oystercatchers (Haematopus ostralegus) are known to feed on diverse intertidal prey such as oysters, mussels, polychaetes, and adjust their diet in response to prey availability and habitat composition. On Yubu Island, previous studies reported shifts in oystercatcher foraging behavior and diet during the breeding season, but direct observation has limitations in accurately identifying prey at the taxonomic level. Recently, fecal DNA metabarcoding has emerged as a powerful approach for precise dietary analysis. This study aimed to identify the prey composition of Eurasian Oystercatchers on Yubu Island using this molecular method. Results: During the breeding seasons of 2015 and 2017, a total of 207 fecal samples were collected near breeding sites. The prey items detected in the feces belonged to 14 orders and 31 genera, including decapods, bivalves, gobies, sipunculans, and isopods. Decapods were the most frequently detected, particularly species from the family Macrophthalmidae. Other major dietary components included bivalves and sipuncula. Comparison with field surveys confirmed decapods such as Macrophthalmus abbreviatus, Macrophthalmus japonicus, and Neotrypaea japonica as the main prey species. Conclusions: Our results highlight that fecal DNA metabarcoding provides more accurate and detailed species-level prey identification than direct observation. Quantifying prey proportions also offered precise insights into oystercatcher dietary preferences. The high prevalence of decapods suggests that exposed mudflats were the primary foraging habitats, reinforcing previous findings that oystercatchers preferentially forage near breeding sites to maximize reproductive success. This study demonstrates the effectiveness of molecular dietary analysis for understanding prey selection and foraging strategies in shorebirds. These findings contribute to a better understanding of the breeding-season foraging ecology of the Eurasian Oystercatcher and support conservation efforts through improved knowledge of prey availability and habitat use.