Steller’s Sea Eagle reproduction ecology. Data of the monitoring of Steller’s Sea Eagle populations on Sakhalin Island and the Lower Amur (Russia)

Study Extent

Most of the data was collected during summer fieldwork. They were made from the beginning of July until the end of August. At this time of the year, nestlings are grown, getting ready to fledge, so it is easier to assess nesting results. In some years, only on Sakhalin, we additionally conducted spring fieldwork; these fieldwork were done in April, when eagles begin to lay eggs.

Sampling

Nesting territories and their status Data were collected once (in the summer) or twice (spring, summer) a year. Spring fieldworks were performed in some of the years, they were done in April, when eagles begin to lay eggs. Summer fieldworks were made from the beginning of July until the end of August. The field works included two main activities: 1) working with nests and territories to estimate the eagles’ productivity and other aspects of breeding performance, and 2) counts of eagles with the purpose of determining the age and territorial structure of the populations. When working on nesting grounds, we inspected known eagle nests and searched for the new ones. For each nest, we determined its status and, if reproduction is going on, the number of fledglings. One eagle pair can build several nests, so the nests were grouped into nesting territories on the basis of territorial proximity taking into account their occupancy status. The doubled radius of the area around the nest that is defended by territory owners, which is approximately 400 m, was used as a general criterion of nest proximity. We also took into account the distribution and territorial behaviour of birds. Nesting territories do not have clear boundaries and are not immutable. Normally, eagle pairs hold the same territories for many years, but sometimes nests can change their belonging to the nesting territory, or a territory can even be split into two pairs, if a new territorial pairs settles in. Bird counts During the fieldwork, we performed counts of the age and territorial structure of the Sakhalin and Lower Amur populations. In addition to special counts, we recorded all occasional sightings of eagles. During 2004–2019 we made more than 7,000 observations of eagles. We aimed to avoid repeated counts within the same year, but obviously the same individuals could be repeatedly met in different years. Adult, immature, and juvenile birds are well distinguished by their age-specific plumage. Also, we recorded the apparent territoriality status of adults: initially we defined it the field by expert judgment, based on the birds’ behaviour (e.g., territorial, defensive, courtship), location relative to nesting areas, and other circumstances of the observation (habitat, number of birds, etc.). Subsequently, these estimates were revised on mapped data, and the final judgment was made. In doubtfulcases, adult birds registered farther than 1.5 km from the nearest sea eagle nest were classified as floaters, the threshold being based on our observations and energetic constraints of the species.

Method steps

1. The database consists of 5 related tables, Nests, Nesting territories, Nest status, Habitats, and Birds Table ‘Nesting territories’ The table ‘Nesting territories’ contains following fields: ID, Longitude (centriod of nests currently included into the nesting territory), Latitude (centriod of nests currently included into the nesting territory), Species (SSE, WTE, or both), District (name of the bay, lake or river), Author, Comment. There are 846 nesting territories of two species of sea eagles including 788 territories of SSE and 58 territories of the White-tailed Eagle (Haliaeetus albicilla, hencefore WTE). These two species sometimes interchange by their nests and nesting territories, so we consider their nests belonging to a common pool. That is why WTE was also included into the database. In Amur study area, there are 350 territories of SSE (including those that belong to both species), and 47 territories of WTE. On Sakhalin, there are 438 territories of SSE (or both species), and 11 territories of WTE. Table ‘Nests’ There are some additional fields: Year_Found, Year_Appearance, Year_Disappearance, Author, Photo_N, Comment and some others. 1556 nests are known on Sakhalin, and 748 nests in Amur. Main fields of the table Nests: Nest_ID, Longitude, Latitude, District, Species (current), Territory_ID, (current)
   Table ‘Nest status’ There are 10080 nest observation records on Sakhalin, and 2488 records in Amur. Main fields of the table are following: Nest_ID, Date, Year, Season (Spring or Summer), Nest_Site_ID, Species, Condition (such as Good, Satisfactory, Destroying, Fallen, etc.), Status (Active, Occupied, Unoccupied), N_nestlings (if status is Active, which means that reproduction is going on), Bear_Press (signs of bear attacks), Adults (number of adults on the nest site), Nest_D_min, Nest_D_max (nest diameter in two dimensions), Comment, Author. Additional fields include information about presence of anti-bear protection and its condition, names of photos, presence of signs of bird activity on the nest (green branches, grass, repaired or not, snow, litter, down, dung, feathers, food remains, etc.) Table ‘Birds’ There are 5272 bird observations on Sakhalin, and 3233 observations in Amur region. Tha main fields are following: Year, Season (Spring or Summer), Date, Longitude, Latitude, Nest_ID (if the bird was met near a specific nest), Territory_ID (if the birds were met on a specific territory), District (bay, lake, river, etc.), Species, Age (Adult, Immature, Fledgling), Status (apparent status of the adult bird, such as Nesting, Floater, etc.), Num_Birds (number of birds observed), Activity (Perching, Ground, Soaring, Flapping flight, etc.), Flush_Distance (for some birds), Author, Comment. Table ‘Habitats’ There are also 275 of habitat descriptions, made on Sakhalin in 2004–2006. They include several scales: 1) nest itself, 2) nesting tree, 3) habitat (vegetation on the nest site), 4) landscape.

1. Haliaeetus pelagicus (Pallas, 1811)
   common name: Steller's Sea Eagle rank: species

There are two study areas: ‘Sakhalin’ and ‘Amur’. The Sakhalin study area stretches from the South to the North, encompassing the coasts of Lunsky, Nabil, Nyisky, Chaivo, and Piltun bays together with the lower reaches of the rivers that flow into these bays. The northern and southern boundaries of the study area correspond to latitudes 53°23'57'' N and 51°11'14'' N, respectively, and the length of the area from the South to the North is approximately 250 km. The eastern boundary coincides with the coastal line, the western boundary lies at a distance of approximately 20–30 km from the coast depending on the hydrological characteristics. Considering only the space that is potentially suitable for sea eagles, the square of the study area is approximately 3280 km2. The Amur study area encompasses the lower reaches of the Amur River together with the channels of the Amur floodplain and associated large and small lakes with rivers flowing into them. The largest lakes are Udyl, Kizi, Kadi, Irkutskoe and Dudinskoe. The northern and southern boundaries of the study area are 139°43'17" N and 139°44'18" N. The western border passes along the river Pilda (139°33'14" E), with the eastern border bounded by the coast of the Tatar Strait. The total area is about 4000 km2.