Background Birds are useful environmental indicators as their presence reflects the health of the food web. Bird occurrence, rarity and abundance are reliable indicators of ecosystem health. Monitoring of avian populations in the Republic of Korea (ROK) is a primary requirement due to plummeting populations and the risks to threatened species. The Ministry of Environment of ROK started conducting winter bird censuses in 1999, including inland areas and coast areas, such as Cheorwon, Yeoncheon, Junam Reservoir and Han River. Cheolwon, Yeoncheon and some extent islands in the West Sea have been survey extensively due to iconic bird species, such as White-naped Crane (Grus vipio) or Red-Crowned Crane (Grus japonensis) wintering there. However, the winter bird census has not covered Yu Islet, Han River Estuary. Yu Islet is located within the Han River Estuary, a protected wetland in the Neutral Zone between the two Koreas and north of Gimpo in the ROK. The Islet currently supports a large, mixed breeding colony of waterbirds, such as one of the nation’s largest concentration of breeding Great Cormorants (Phalacrocorax carbo) and smaller numbers of breeding Black-faced Spoonbill (Platalea minor), Grey Heron (Ardea cinereal), Great Egret (Ardea alba) and Intermediate Egret (Ardea intermedia). Access to the area has long been restricted for military reasons, but recently, regular survey activity is possible supported by Gimpo City and the military base in Gimpo from November 2018. New information Here, we provide data demonstrating that Yu Islet is important for breeding for waterbirds; and that the northern Gimpo part of Han River Estuary is also internationally important for waterbirds during the migratory bird season, as defined by the Ramsar Convention (Ramsar 1971, RRC-EA 2017). In particular, four waterbird species were found during the survey in the Main Survey Area: Swan Goose (Anser cygnoides), Taiga Bean Goose (Anser fabalis), Tundra Bean Goose (Anser serrirostris) and Greater White-fronted Goose (Anser frontalis). Once considered widespread in East Asia and abundant, the world population of Swan Goose is now estimated at only 60,000 - 78,000 individuals (Wetlands International 2020) and the species is assessed by BirdLife International as globally Vulnerable (BirdLife International 2020). The 1,010 Swan Goose (Anser cygnoides) counted on the vegetated mudflats at Jogang-Ri in the Main Survey Area on 27 November 2018 represents more than 1% of the total world population of this species. Notably, it is also the highest count of this species in the ROK for at least a decade. The count confirms the continuing international importance of the Han River Estuary for the survival of the Swan Goose. The number counted in November had fallen to 250 by 28 December 2018; and none was recorded in the Main Survey Area in January or February 2019. Although searched for in March, none was noted during the northward migration either. The surveys also found small numbers of nationally-scarce Grey-capped Pygmy Woodpecker (Yungipicus canicapillus) in several areas of woodland surrounded by the Han River Estuary. By selecting the most species-rich count within a given month in each of the two survey sectors, the number of species we recorded ranged from a minimum 29 in January to a maximum of 65 in April 2019. Based on the species recorded, the survey area is clearly important for avian conservation. Its importance derives from the combination of the extensive areas of high-quality wetland and its geographic location within one of the Korea Peninsula’s largest and most important remaining wetland ecosystems, the Han River Estuary. Our surveys resulted in the detection of a substantial number of bird species, especially in March and April when forest-breeding birds are more obviously vocal. The survey result is provided in the supplementary material (Suppl. material 1).

中文翻译：

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韩国内陆边境地区鸟类调查，大韩民国金浦

背景 鸟类是有用的环境指标，因为它们的存在反映了食物网的健康状况。鸟类的出现、稀有和丰富度是生态系统健康的可靠指标。由于种群数量锐减和对受威胁物种的风险，对大韩民国 (ROK) 的鸟类种群进行监测是一项主要要求。韩国环境部于 1999 年开始进行冬季鸟类普查，包括内陆地区和沿海地区，如铁原、涟川、朱南水库和汉江。由于标志性的鸟类物种，如白枕鹤 (Grus vipio) 或丹顶鹤 (Grus japonensis) 在那里越冬，因此对铁原、涟川和西海的某些岛屿进行了广泛的调查。然而，冬季鸟类普查尚未覆盖汉江口玉岛。鱼岛位于韩江河口，是两韩之间的中立地带和韩国金浦以北的受保护湿地。小岛目前支持一个大型的混合繁殖水鸟群，例如全国最大的繁殖大鸬鹚 (Phalacrocorax carbo) 和少量繁殖的黑脸琵鹭 (Platalea minor)、灰鹭 (Ardea cinereal)、大白鹭（Ardea alba）和中白鹭（Ardea intermedia）。由于军事原因长期以来一直限制进入该地区，但最近，从 2018 年 11 月起，金浦市和金浦军事基地可以支持定期调查活动。水鸟；并且汉江河口的金浦北部部分在国际重要的候鸟季节对水鸟来说也是重要的，如拉姆萨尔公约所定义（拉姆萨尔 1971 年，RRC-EA 2017 年）。特别是，在主调查区调查期间发现了四种水鸟：天鹅鹅（Anser cygnoides）、针叶鹅（Anser fabalis）、苔原豆鹅（Anser serrirostris）和大白额鹅（Anser frontalis）。曾经被认为在东亚广泛分布和丰富的天鹅鹅的世界种群现在估计只有 60,000 - 78,000 只（湿地国际 2020），该物种被国际鸟类保护协会评估为全球易危物种（国际鸟类保护组织 2020）。1, 010 天鹅鹅（Anser cygnoides）于 2018 年 11 月 27 日在主要调查区 Jogang-Ri 的植被泥滩上进行计数，占该物种世界总人口的 1% 以上。值得注意的是，它也是韩国至少十年来该物种的最高数量。计数证实了汉江河口对天鹅生存的持续国际重要性。到 2018 年 12 月 28 日，11 月统计的人数已降至 250 人；2019 年 1 月或 2 月在主调查区没有记录。虽然在 3 月进行了搜索，但在北移过程中也没有发现。调查还发现，在汉江河口环绕的几个林地地区，有少量全国稀缺的灰顶侏儒啄木鸟（Yungipicus canicapillus）。通过在两个调查部门中选择给定月份内物种最丰富的计数，我们记录的物种数量从 2019 年 1 月的最少 29 个到 2019 年 4 月的最多 65 个不等。根据记录的物种，调查该地区显然对鸟类保护很重要。它的重要性来自于大面积优质湿地及其在朝鲜半岛最大和最重要的湿地生态系统之一汉江河口内的地理位置的结合。我们的调查发现了大量的鸟类，特别是在三月和四月，当森林繁殖的鸟类更明显地发出声音时。调查结果在补充材料（补充材料 1）中提供。我们记录的物种数量从 2019 年 1 月的最少 29 种到 2019 年 4 月的最多 65 种不等。根据记录的物种，调查区域显然对鸟类保护很重要。它的重要性来自于大面积优质湿地及其在朝鲜半岛最大和最重要的湿地生态系统之一汉江河口内的地理位置的结合。我们的调查发现了大量的鸟类，特别是在三月和四月，当森林繁殖的鸟类更明显地发出声音时。调查结果在补充材料（补充材料 1）中提供。我们记录的物种数量从 2019 年 1 月的最少 29 种到 2019 年 4 月的最多 65 种不等。根据记录的物种，调查区域显然对鸟类保护很重要。它的重要性来自于大面积优质湿地及其在朝鲜半岛最大和最重要的湿地生态系统之一汉江河口内的地理位置的结合。我们的调查发现了大量的鸟类，特别是在三月和四月，当森林繁殖的鸟类更明显地发出声音时。调查结果在补充材料（补充材料 1）中提供。它的重要性来自于大面积优质湿地及其在朝鲜半岛最大和最重要的湿地生态系统之一汉江河口内的地理位置的结合。我们的调查发现了大量的鸟类，特别是在三月和四月，当森林繁殖的鸟类更明显地发出声音时。调查结果在补充材料（补充材料 1）中提供。它的重要性来自于大面积优质湿地及其在朝鲜半岛最大和最重要的湿地生态系统之一汉江河口内的地理位置的结合。我们的调查发现了大量的鸟类，特别是在三月和四月，当森林繁殖的鸟类更明显地发出声音时。调查结果在补充材料（补充材料 1）中提供。