Effects of timing of ice melt on spring stopover patterns of migrating Whooper Swans Cygnus cygnus and Bewick’s Swans Cygnus columbianus bewickii in the Russian part of the eastern Gulf of Finland

Abstract

Seasonal climate dynamics have a substantial impact on the timing of optimal feeding conditions for migratory birds at their stopover sites in the temperate and boreal zones. Delayed spring events (such as the timing of ice melt, and thus availability of open water) at stopover sites may result in migratory flocks forming at later dates. On these occasions, birds may be faced with the choice of maintaining their usual stopover duration, to make full use of the food resources needed to complete migration and clutch formation, or of increasing the overall speed of migration by shortening or skipping some of the stopovers, in order to arrive at their breeding sites on time. Additionally, it may be possible that the birds use alternative, non-traditional stops in such years. The purpose of this study was to determine the effect of the timing of ice melt at a major spring staging site on the migration patterns of swans. Long-term (2005–2021) censuses were conducted in an area supporting the largest concentrations of migratory waterfowl in the eastern part of the Gulf of Finland (Kurgalsky Peninsula: 59.725°N, 28.118°E). Late ice melt resulted in a reduction in the migration duration and peak abundance of both Bewick’s Swans Cygnus columbianus bewickii and Whooper Swans C. cygnus. The date on which the staging period ended however varied little from year to year. Use of alternative stopover sites on agricultural land by the swans was recorded only in the most recent years, when spring was early.