Migration behaviour of long- and medium-distance songbird migrants

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Birds can be distinguished between residents and facultative and obligate migrants, although these categories are not distinct, and their extremes are set at different ends of a continuum. Within the Palaearctic-African migration system, migrants are separated into trans-Saharan migrants, i.e., species that winter south of the Sahara, which show longer seasonal migration periods (long-distance migrants), and those that winter farther north (medium-distance migrants). Due to our current lack of fundamental information about the potentially different migration strategies of these two migration groups, we lack useful explanations of why the changes in the seasonal phenology of long-distance migrants that occurred in response to changes in climate differed from those of medium-distance migrants. Although several studies have examined the migration behaviour of single species of both of these groups, to date, no study has investigated potential differences in their migration behaviour by jointly considering species of both groups at the same time using an experimental approach and under free-flying conditions. Optimal migration theory predicts that medium-distance migrants have more time for their seasonal movements than long-distance migrants. Therefore, the former is predicted to have more time for travelling to minimize energy expenditure. In contrast, long-distance migrants are supposed to maximize the speed of migration to reach their more distant destinations in time. Because longer migration distances are more costly in terms of time and energy, long-distance migrants have less time and energy for other life history traits, i.e., breeding, moulting, and wintering. Therefore, stronger selection pressure is predicted to act on species with longer migration routes compared with those with shorter routes. The small degree of phenotypic variation that has been observed in the seasonal temporal organization of life-history events in long-distance migrants was hypothesized to come from low levels of heritable variation. Here, I propose to critically study the migration behaviour of long- and medium-distance migrants using four comparable study species. To do so, I will combine cage experiments involving temporarily confined migrants, for which environmental conditions can be controlled, with studies of free-flying birds on Helgoland. To estimate the migration behaviour of free-flying birds, I plan to set up a large-scale automated digital telemetry array in addition to the existing telemetry station on Helgoland. With this system, birds' movements across large parts of the German Bight and the adjacent coastal areas would be covered. This would be the first study to demonstrate, using a holistic approach, the different solutions found by selection for long- and short-distance migrants in terms of both their migration behaviour (time vs. energy) and their range of reaction norms to environmental variation during both spring and autumn migration.