Seasonal distributions of Caspian Terns Hydroprogne caspia from Swedish populations, revealed by recoveries and resightings of ringed birds

Abstract

The aim of this study was, by analysing recoveries and resightings of Caspian Terns Hydroprogne caspia ringed in Sweden, (1) to identify areas of importance during their migration and wintering, (2) to investigate the annual routines of migration and wintering of different age categories, and (3) to investigate how human activities, especially shooting, have affected survival of Caspian Terns over the 80-year period. In total, 2,048 metal-ring recoveries and resightings from 1,420 birds and 101 colour-ring recoveries and resightings since 2011 from 85 Caspian Terns ringed in Sweden were analysed for spatio-temporal patterns, age-dependent differences and mortality due to human activities. Ring recoveries beyond the breeding areas were widely dispersed southward but concentrated in several locations, such as coastal areas in the Netherlands, Italy and Germany during migration seasons and in Mali and Tunisia during winter. Distances to their wintering areas of up to more than 6,000 km were longer than for other populations of this species. Age-dependent differences in seasonal distributions seemed to be due mainly to young Caspian Terns remaining in wintering areas or otherwise well south of the breeding areas during their first years of life. Although shooting appeared to have been an important cause of death in former years, it has declined and recently seems to have had less influence on survival of European Caspian Terns than in the 1940s–2000s. Other recent changes in the distribution of recoveries are also discussed from ecological and social perspectives.