Millions of birds encounter ecological barriers, such as oceans and deserts, during their migratory journey every year. These ecological barriers force the birds to somehow change behaviour in order to pass the barrier and continue their migration, and a successful barrier passages are often a question of life and death. Today, many long-distance migratory birds are declining. These species often face the longest barrier crossings along their journey, and their success in these enterprises depend on predictable resources for fuelling before and after the barriers, possibly making them more vulnerable to global change. Migrating birds are accordingly considered of extra importance in the EU directive 2009/147/EC on the conservation of wild birds. Advances in the ability to track birds during the last decades have reveal¬ed previously unknown spatiotemporal pat¬terns of bird migration. However, previous studies were limited by loggers that only collected data with rather poor spatial and temporal resolution, or by methods only collecting data at the local scale (such as radar and radiotelemetry). Hence, individual migratory performance throughout the journey, the strategies and resulting outcomes of migratory behaviour, still remains largely unknown. This action aimed to use state-of-the-art tracking technology to study long-distance migratory songbirds as they cross barriers during migration. By understanding birds behaviours at barrier crossing, and by understanding how plastic these behaviours are and if and how they differ from other parts of the migratory journey, the goal is to outline the importance of barriers to bird migration. Preliminary data from the project reveals that birds do behave differently at barrier segments compared to other migratory segments. Nocturnally migrating species shift behaviour when crossing inhospitable areas, and sometimes instead fly both at night and day to pass the area as fast as possible. However, the behavioural change related to the crossing of barriers largely depends on the biology of the species. An extreme altitudinal behaviour is also associated with barrier crossings, this behaviour is possibly shown to conserve heat balance throughout the flight, but can probably also limit the individual birds’ performance if the bird fails to regulate flight altitude appropriately. Furthermore, tracking data from displaced common cuckoos indicate that juvenile cuckoos, similar to adult cuckoos, are able to use true navigation.
