Population, Breeding Phenology, and Factors Affecting Breeding Success of River Tern (Sterna aurantia) in Eastern India

Abstract. Although the River Tern (Sterna aurantia) has been widely distributed across south and southeast Asia, its population is declining and was classified as Vulnerable in the 2020 IUCN Red List of Threatened Species. Insufficient information is available on its breeding ecology. This study reports the breeding ecology and factors affecting breeding of the River Tern over three years (2017-2019) along the Mahanadi River in Odisha, eastern India. Breeding was observed between February and May, with peak activity in April. The mean clutch size was 2.4 ± 0.06 SE and incubation period was 22.1 ± 0.06 days. Out of 163 nests, 68 nests succeeded in hatching at least one egg and apparent nesting success rate varied between 2.6% (2018) and 62.3% (2017). Factors affecting nesting success (95 nests) were predation (37.9%), flooding (32.6%), a cyclone (28.4%) and nest desertion (1.1%). Overall, of 124 chicks hatched, 58 were estimated to have fledged. Factors affecting chick survival were a cyclone (43.9%), predation (39.4%), and unknown causes (16.7%). A cyclone in May 2019 wiped out all eggs and chicks. Development and implementation of an evidence-based multi-species conservation action plan to protect River Tern and other globally threatened riverine ground nesters is urgently required.