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The Importance of Barriers to Bird Migration

Periodic Reporting for period 1 - BIRDBARRIER (The Importance of Barriers to Bird Migration)

Reporting period: 2018-09-01 to 2020-08-31

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.
Three different species of songbirds have been tracked with activity loggers in order to map their behaviour throughout the annual cycle. Red-backed shrikes Lanius collurio, and great reed warblers Acrocephalus arundinaceus are long-distance migratory songbirds migrating between their breeding areas in Palearctic and wintering areas in Africa every year. While the red-backed shrikes prefer open farmland and bushland and is a carnivorous species feeding on a wide variety of prey from insect to small amphibians and mammals, the great reed warblers are strictly reed living and feeding preferably on insects in reed beds. Preliminary tracking results show that both species sometimes prolong flights into daytime when crossing oceans and desert but that this also is depending on the biology of the species. These prolonged flights are associated with a specific altitudinal behaviour, possibly to keep the heat balance throughout daytime flights. Both species fly generally higher than previously expected with substantial climbs and descends throughout the migratory flights and the highest altitudes (of more than 6000 m asl) are found at barrier crossings underlining the importance of these flight segments.

Snow buntings Plectrophenax nivalis are the northernmost breeding songbird and the population studied within this project breeds at Svalbard and winter on the plains south of the Ural Mountains. Unlike red-backed shrikes and great reed warblers, snow buntings commonly migrate in flocks and the diurnal schedule of their migration have been discussed (diurnal vs nocturnal flight schedules). Preliminary tracking results shows that the journey back and forth to Svalbard from mainland Scandinavia includes flights on more than 24 h over open Sea and a mixed diurnal and nocturnal flight strategy. The snow buntings also migrate at lower altitudes compared to red-backed shrikes and great reed warblers, possibly as a result of their northern range.

Another aspect of crossing a major barrier is how do birds establish orientation during long flights over areas that lack perceivable landmarks? Songbirds are known to be able to use several different cues to decide migratory directions, among them a magnetic inclination compass, but how and when they use this information during migration is so far unknown. Juvenile birds inherit a migratory direction and distance, and are capable of orientating (finding a direction and keeping it). Adult birds have been shown to be able to correct for displacement during migration, and find the direction to a known area from an unknown position (commonly referred to as true navigation). In order to understand if juvenile birds have the same ability as adult birds to use true navigation, juvenile and adult common cuckoos Cuculus canorus were caught on migration in Rybachy, Kaliningrad, displaced 1800 km east to Kazan, Russia, and released with GPS transmitters. After the displacement both juveniles and adults corrected for the displacement and flew westwards to get back to their natural migratory route, indicating that juvenile cuckoos are, similar to adult cuckoos, able to use true navigation.
This project underlines the complexity of migration, discovers novel and extraordinary migratory performance and highlights how much we still do not know about the remarkable journeys of long-distance migratory birds. A previously undescribed behaviour, that represents a new perspective on the critical importance of heat dissipation during bird flight, clearly shows that understanding flight altitudes is essential to understand physical capabilities of migrants and crucial to fully understand migration flight strategies and the costs of migratory flights. Moreover, results from the project provides a new hypothesis for why the overwhelming majority of non-soaring long-distance migrant birds generally conduct their impressive migratory flights during the night, i.e. at a time when solar radiation is absent. In conclusion, the project has revealed astonishing adaptations needed for a small songbird to successfully conduct long-distance migratory flights over inhospitable areas, and has underlined how vulnerable these birds are to changing climate and habitat conditions, such as increased desertification. It is also clear that it is crucial that we understand not only where birds go, but also how they get there, for successful conservation of long-distance migratory birds.
Red-backed shrike Lanius collurio released after being euipped with an activity logger.