Introduction

The black-tailed gull (Larus crassirostris) is a common seabird distributed in East Asia, including Korea, eastern China, and Japan (Park, 2014). In Korea, it is known to be a common resident bird that breeds in groups on uninhabited islands such as Dokdo, Hongdo, Chilsando and Nando (Kang et al., 2008; Kim et al., 2017; Kim et al., 2022). Black-tailed gulls are top predators of the food chain. As representative marine birds, they are also used as pollutant monitoring organisms (Lee et al., 2017; Choi et al., 2018). Black-tailed gulls have the characteristic of breeding in groups. Various studies have been conducted on their breeding status, growth rates, breeding population, home range, survival and return rates, habitat selection, and so on (Kwon & Yoo, 2007; Lee et al. 2008; Kang et al., 2008; Kim et al., 2022, Lee S.H. et al., 2023). Black-tailed gulls are coastal birds with different breeding and wintering habitats. After breeding, they disperse inland for the winter and show various migration patterns according to their wintering habitats (Yu et al., 2006; Xia. et al., 2023). Regarding the migration of black-tailed gulls in Korea, there are results showing that they have moved from Baengnyeongdo (in the West Sea) to China (NIBR, 2019). Studies on the annual range of black-tailed gulls on Dokdo and Hongdo have found that individuals breeding on Dokdo in winter in Japan and individuals breeding on Hongdo migrate to the southern coast area in Korea and even Japan (Hong et al., 2019; Kim et al., 2021). On the west coast of Korea, Chilsando, Bulmudo, Nando, and Baengnyeongdo are breeding grounds for black-tailed gulls, with about 50,000 individuals breeding on Chilsando (Lee et al., 2017; Kim et al., 2022; Lee et al., 2022; Lee & Lee, 2022). However, the wintering population of black-tailed gulls in Korea is estimated to be approximately 55,000 in 2021-2022 (NIBR, 2022). Monthly population fluctuations are also reported in the Nakdong River Estuary, a major habitat (Hong & Lee, 2011). Seasonal migration of black-tailed gulls on Dokdo and Hongdo known to be their breeding grounds has been partially conducted. However, there is a lack of information about black-tailed gulls’ habitat use in the southwest coast near Chilsando, a large breeding site. Therefore, this study analyzed characteristics of wintering habitat use of black-tailed gulls near Chilsando on the southwest coast of Korea based on GPS tracking data.

Materials and Methods

Results

Discussion

Most black-tailed gulls on the southwestern coast of Korea migrate to their wintering grounds in December. They can be divided into three groups based on their major wintering grounds: Group 1, southwest coast island area (Heuksando, Chujado, Gageodo, Jejudo etc.); Group 2, inland coastal area; and Group 3, China. Migration to wintering grounds begins in September on Hongdo (southeast coast), July and August on Dokdo (east sea), and begins southward migration in November on Xingrentuo Islet, China (Hong et al., 2019; Kim et al., 2022; Xia et al., 2023). In this study, most black-tailed gulls on the west coast migrated to their wintering grounds in December, showing a similar tendency to black-tailed gulls breeding on Xingrentuo Islet, China. Black-tailed gulls show many different distances and patterns of migration to their wintering ground (Hong et al., 2019; Kim et al., 2022; Xia et al., 2023). Winter migration of black-tailed gulls is highly correlated with temperature drop. Marine environment (food resources) and human activities (fishing boat) are also important factors affecting their winter migration (Rock, 2016; Hong et al., 2019; Kim et al., 2022; Xia et al., 2023).

Black-tailed gulls on the southwest coast lived mainly in ports on the inland coast in October, showing a narrow range of action and a low flight rate. Their post-breeding period is an important energy storage period for migration. Movement is minimized during this period. It is known that the same habitat as the breeding season is used when food is abundant or a resting place is provided (Kim et al., 2022; Xia et al., 2023). Therefore, October is the post-breeding period. The inland coastal port area is thought to provide abundant food and resting places, resulting in a narrow home range and a low flight rate. From November, it was confirmed that they were moving more than 100 km into the sea around the coastal port. Some individuals used waters of the southwest coast island.

Black-tailed gulls use anchovie (Engraulis japonicus), Pacific sand lance (Ammodytes personatus), and shrimp (Amphipoda) as food sources (Kim et al., 2017). They also eat invertebrates and shellfish. They follow fishing boats (Lee S.H. et al., 2023; Xia et al., 2023). The reason for the increase of home range from November might be due to the distribution of food resources and the increase in fishing activities in the west coast. In December, island areas were widely used, focusing on Chujado and Gageodo on the southwestern island areas. Some individuals also used inland port areas (Fig. 2). The reason why black-tailed gulls widely use island areas of the southwest coast in December is also thought to be result of the distribution of food resources and the increase in fishing activities by fishing boats (Acros & Oro, 1996; Lee et al., 2023; Xia et al., 2023). In addition, this movement is considered an activity to explore wintering areas, such as checking food resources and resting places to select a wintering area (Xia et al., 2023). However, some individuals persisted in inland coastal ports. The reason why they continue to use the inland coast in December might be because they want to stay as long as possible in a familiar environment with abundant food resources and a place to rest (Xia et al., 2023) and then move when the temperature drops. The main use areas in January and February showed clear differences by group, with Group 1 using the southwest coastal island area, Group 2 using the inland port area, and Group 3 using China. All flight rates showed a decrease. Black-tailed gulls feed during day and night. Their main feeding areas include various areas such as ports, coasts, oceans, residential areas, commercial areas, and rice fields (Yoda et al., 2012; Rock et al., 2016; Lee S.H. et al., 2023). Therefore, the reason why black-tailed gulls have a very narrow home range and flight rate during the main wintering season is thought to be the result of choosing a wintering strategy of opportunistic foraging through day and night flights in various environments during the wintering season. In March, they return to their breeding grounds. Their home range and flight rate are increased. When black-tailed gulls return to their breeding grounds in March, they return to the same breeding grounds and make exploratory movements to search for food resources and select a mate during the pre-breeding period (Xia et al., 2023), which might have increased their home range and flight rate.

In this study, black-tailed gulls showed a narrow home range in October before moving to their wintering grounds. Their home range increased from November, showing the widest home range in December. These results were similar to results of a previous study showing that black-tailed gulls had a narrow home range in their wintering grounds on the west coast (Lee S.H. et al., 2023). However, these results were different from other research studies showing that the non-breeding season had a narrower right of action than the breeding season (Hong et al., 2019; Kim et al., 2022). Since home range during the breeding season was not analyzed in this study, the difference between breeding and non-breeding seasons could not be clearly confirmed. Thus, follow-up research should analyze their home range during the breeding season.

Black-tailed gulls show high returning to the same breeding site (Hong et al., 2019; Kim et al., 2021; Seo et al., 2023). They have unique genotypes according to their breeding sites (Jeong et al., 2018). However, the genetic diversity of domestic black-tailed gulls is moderate. Active gene flow between breeding populations has been reported (Lee Y.S. et al., 2023). Therefore, it is difficult to determine whether the difference in wintering distribution areas between east and west coasts is due to genetic differences between breeding groups. Follow-up research is necessary. There were differences in migration time and migration patterns between black-tailed gulls on east and west coasts. These results are thought to reflect wintering characteristics of the west coast population, environmental factors such as the marine environment and fishing activities, and geographical characteristics of the west coast, where many islands are distributed. However, black-tailed gulls sometimes choose different wintering sites every year. They also choose different habitats for wintering, pre-breeding, breeding, and post-breeding (Xia et al., 2023). Continued research on habitat use characteristics among breeding groups of black-tailed gulls will be needed in the future.