Nature reserve "Lebjazhyi" is one of the most important migration stopovers of Bewick's swans (Cygnus bewickii) on the Russian part of the Gulf of Finland. The aim of the study was to identify the main feeding areas for Bewick's swans there, to determine the species of feeding plants and to define whether the identification of swan’s feeding areas is possible on satellite images.
Route counts of swans were conducted during the spring migration. GPS, rangefinder and the grid of poles helped to determine the exact coordinates of the birds on the water. Feeding or non-feeding was recorded as the type of behavior activity. The distribution of two dominant species fennel pondweed (Potamogeton pectinatus) and green algae Cladophora (Cladophora glomerata) was studied in summer because pondweed sprouts appear after completion of the swan’s migration.
Cladophora dominated among the underwater vegetation in the reserve. However, the distribution of the swans on the territory did not depend on the distribution of Cladophora. As a result of the analysis, it was shown that feeding swans concentrated only in those areas where fennel pondweed grows. The biggest concentrations of swans were observed in the areas with the biggest concentrations of fennel pondweed. Such dependence hasn’t been identified for non-feeding birds. They were evenly dispersed. Thus it has been suggested that the swans feed fennel pondweed in the area of the study. This plant is a typical food for swans. In spring birds found in the bottom starchy tubers of fennel pondweed and feed them during migration.
Geobotanical investigations revealed that pondweed grows among the patches of reeds and rushes, which are easily identified on the satellite images. Moreover, the patches of underwater plants differ in color characteristics on the images. The boundary of the swan’s feeding area drawn on the image before the field studies exactly coincided with the pondweed growth zone and distribution of feeding swans.