A lake wetland ecosystem consists of shallow water, marshes, mudflats and sparse grass flats that are natural habitats and feeding grounds for migratory birds and other wetland-dependent wildlife, especially some endangered species (Paszkowski and Tonn 2000, Lyche-Solheim et al. 2013, Wang et al. 2019). The middle and lower reaches of the Yangtze River Basin have the most representative and largest concentration of freshwater lakes in China and provides an important habitat for globally significant numbers of birds (Zhang et al. 2022). However, intensive human activities (agriculture, urbanisation, land reclamation and conversion) and anthropogenic factors (global climatic variation and flooding) have led to the reduction of the lake area and ecological degradation, making the middle and lower reaches of the Yangtze River Basin one of the most endangered areas in China (Cui et al. 2013, Wang X et al. 2020).

Since the 20^th century, the Chinese government has taken active measures to protect wetlands. At present, a wetland protection network system has been basically formed with wetland nature reserves as the main body and internationally important wetlands and wetland parks as a combination, which have become an effective measure to protect endangered and rare birds (Zheng et al. 2012). Nature reserves have been established in important lakes, such as Poyang Lake and Dongting Lake and systematic bird research has been carried out in these areas. For example, Debela et al. (2020) have clarified the composition and diversity of the over-wintering aquatic bird community in three nature reserves around Poyang Lake. Yang et al. (2016) have explored the spatiotemporal pattern of bird habitats in Poyang Lake. Zhu et al. (2021) have studied habitat suitability of migratory birds to the water level fluctuations on Dongting Lake.

Hongze Lake is the fourth largest freshwater lake in China and the largest water storage lake along the Eastern Route of the South-North Water Diversion Project (Yang et al. 2021, Zhu et al. 2022). Sihong Hongze Lake Wetlands National Nature Reserve (SHLWNNR) is located at the northwest of Hongze Lake, which is the most completely preserved and representative reserve in China with the breeding of plentiful biological species (Qin et al. 2020). The total area of the Reserve is 49365 hm², the wetland types mainly including lake wetlands, marsh wetlands and river wetlands, accounting for 92.26% of the total area of the Reserve (Ye et al. 2004). In addition, the Reserve is located in a dense intersection area of the East Asian-Australian Flyway (EAAF), which is an important stopover site and overwintering ground for migratory birds and a crucial area for the protection of rare and endangered bird species (Jiang et al. 2017).

Bird studies in SHLWNNR are generally lacking and available reports are not specific and incomplete. For instance, Wang et al. (2014) conducted a comprehensive scientific expedition to the Reserve and recorded 147 bird species, belonging to 15 orders and 47 families. The management office of the Reserve prepared the "2016-2019 Bird Survey Report of Sihong Hongze Lake Wetlands National Nature Reserve", based on the observations and records from 2016 to 2019. From the results of the four-year bird survey, a total of 207 species of birds were recorded, belonging to 14 orders and 53 families. As of July 2021, a total of seven bird observation reports have been collected from the Reserve in the China Bird Report (http://www.birdreport.cn/), belonging to 84 species of birds in 14 orders and 34 families. Wang et al. (2022) discussed the population distribution and interannual variation of Anatidae in the Reserve during the wintering period and showed that there were 24 Anatidae species with the number of recorded species and population increasing year by year. Qin et al. (2020) studied different vegetation planting schemes to promote the protection of typical summer migratory birds (Chlidonias hybrida (Pallas, 1811)) and winter migratory birds (Fulica atra Linnaeus, 1758) in the Reserve. However, these studies recorded and published bird data for only some months or for some species and the research of overall bird diversity are from many years ago. Therefore, we conducted a more comprehensive and systematic field survey to publish an occurrence dataset for updating the bird species list and identifying the status of bird resources in the Reserve throughout the year. It also provides basic data for subsequent researchers to conduct bird research and endangered species conservation in the future.

This research was supported by the Jiangsu Province Important Wetland Flora and Fauna Diversity Monitoring Project (Project No. 202004120). We are grateful to Manyu Zhang, Taiyu Chen, Silu Wang and Mengmeng Liu for participating in the field investigation. This research was also assisted by the Administrative Office of Sihong Hongze Lake Wetlands National Nature Reserve.

Huali Hu - Data preparation, GIS processing, manuscript editing; Wei Hu - Species identification; Zheping Xu - Review and upload data to GBIF; Changhu Lu - Review and editing