Adaptive flexibility in migratory behaviour of shorebirds

Abstract: The phenomenon of bird migration has long fascinated us humans and it has prompted questions like where do they go or come from and how do they find their way? Arctic shorebirds are well known for their spectacular long-distance migrations with some species travelling more than 10 000 km every spring and autumn, and some even cover this distance in one single nonstop flight. The aim of my thesis was to investigate the flexibility of Arctic shorebirds when it comes to orientation behaviour, departure behaviour and migration strategy. In spite of the extensive knowledge about orientation and migration performance of birds there is still a poor understanding of how birds orient when actually travelling on migration. Flight directions and routes of migrating birds are determined by the birds’ compass orientation, but also by effects of wind, social influence, responses to topography and landmarks, and to navigation cues. I found fine-scaled orientation changes of migrating shorebirds passing southern Sweden. The shorebirds might travel approximately along a straight geographic compass course, with deviations due to large-scale topography to maximize general association with coastal habitats during migration. Younger birds may learn the general flight paths and course changes from older and experienced individuals in the flocks, but most of this learning process between generations probably does not take place until after the birds’ first autumn migration since juveniles migrate at a later time in the season than adults. I also found that the shorebirds showed clearly different responses to crosswinds when passing southern Sweden demonstrating that shorebirds are remarkably variable in their response to crosswinds also during short sections of their migratory journey. I compared departure and flight activity of shorebirds migrating in contrasting wind regimes during autumn (high probability of wind resistance) and spring (high probability of wind assistance) in southern Scandinavia. The shorebirds changed their threshold for departure in relation to wind between the two seasons in a dramatic way: flying almost exclusively with wind assistance during spring migration but regularly with wind resistance during autumn. The degree of wind selectivity in relation to the distributions of available wind effects was similar during autumn and spring, during both seasons the shorebirds selected to migrate on days with better winds than expected. My results show that birds do not have a fixed threshold of wind assistance for their departure on migratory flights but the same birds depart in very different wind situations during spring and autumn seasons in southern Scandinavia when winds regimes in relation to the birds’ migratory direction were very different. This suggests that migratory birds have an adaptive flexibility in their responses to wind depending on seasonal wind regimes. Finally, I investigated stopover lengths and flight distances of dunlins Calidris alpina migrating along the East Atlantic flyway to characterize the autumn migration strategy and compared these results with the migration strategy used in spring along the same route in order to understand the evolutionary and ecological causes and consequences of flexibility in migration strategy. The autumn migration strategy of the dunlins was characterized by short-stop-short-hop migration for both adults and juveniles while during spring, dunlins adopted a long nonstop migration strategy instead. The major reasons for these contrasting strategies between seasons are probably food availability and wind pattern. In conclusion my thesis shows that arctic shorebirds and probably other migratory birds have an impressive flexibility when it comes to migratory behaviour and in many cases this flexibility is adaptive.