From the Alps to Africa: ecology and migration of Northern Wheatears (Oenanthe oenanthe) breeding at high elevation, with insights into an intra-tropical migratory system

The populations of Afro-Palearctic migratory birds are declining. These species are more sensitive to climate change than short-distance migrants or residents. Because mountain ecosystems are under increased pressure of climate and land-use changes, I investigated one of the few long-distance migrants breeding at high elevations in the Alps, the Northern Wheatear Oenanthe oenanthe. The first three chapters of this thesis are focusing on breeding-site ecology in the Swiss Alps and on the avifauna of alpine grasslands. In chapters four and five, I investigated migration of the Northern Wheatear. In the sixth chapter, I broadened the scope by studying a different migration system from the southern hemisphere.
In the first chapter, I identified drivers of nest-site selection and breeding success. Nests were more likely located at lower elevation on gentle, south-exposed slopes. Productive pastures interspersed with rockpiles and bare ground were preferred. As Wheatears are territorial, the surroundings of conspecific nests were avoided for selecting a nest site. Predation was the main driver of breeding failure. Breeding success seemed similar among years with strong inter-annual variation in spring onset, to which laying dates were adjusted.
In the second chapter, the focus shifted to foraging micro-habitat. My colleagues and I observed ringed Northern Wheatears between May and September. Wheatears preferred rocks and short vegetation throughout the presence at the breeding site. The snow front was favoured for foraging only upon arrival from migration, whereas bare ground played an important role while provisioning food for the chicks when the grass layer was high, and after the breeding period. Wheatears preferred the vicinity of burrows of Alpine Marmots Marmota marmota, where bare ground was available. The preference for heterogenous habitats on a small scale suggests sensitivity of Northern Wheatears to landscape homogenization due to intensification, or to bush encroachment as a consequence of land abandonment.
In the third chapter, I extended my research beyond the Northern Wheatear. I considered fine-scale and landscape-scale habitat preferences of co-occurring insectivorous species inhabiting alpine grasslands by conducting repeated surveys. The Water Pipit Anthus spinoletta, an alpine specialist, had higher densities on gentle, productive slopes, and lower densities in dense shrubs. Wheatears preferred open, south-exposed and gentle slopes with human-made rockpiles. While Wheatears avoided shrubs, the closely related Whinchats Saxicola rubetra, a species of conservation concern in Europe, preferred this habitat and had higher densities at lower elevations close to the treeline. Our results underline the importance of high-elevation pastures for the conservation of species that are declining in Europe, as long as low-intensity management maintains patchy habitats.
The following chapters focused on migration, starting with a comparative study in which migration routes and timing of Northern Wheatear populations in the Swiss and Austrian Alps were described. Using light-level geolocators, my co-authors and I compared their migration with a lowland population in Germany. Lowland birds returned earlier compared to birds from the Alps where spring onset is delayed. Wheatears from Switzerland and Austria often directly crossed the Mediterranean Sea, while birds from Germany migrated towards the Iberian Peninsula. All three populations overwintered in the Western Sahel.
In the fifth chapter, I used multi-sensor geolocators to investigate the movements of Northern Wheatears with unprecedented spatial and temporal resolution. Stationary sites were located with enhanced precision compared to light-level geolocation, by correlating the air pressure measured on the birds with global atmospheric pressure data. I identified the role of islands as optional stopover sites while crossing the sea, and I located the main stopover areas between the Mediterranean and the Sahara in the Atlas Mountains. Furthermore, atmospheric pressure data informed on flight altitudes, highlighting longer, faster, and higher flights above the sea and the desert as barrier-crossing behaviour. When the birds returned in May, local altitudinal movements towards the Alpine valley took place in response to unfavourable weather events with snowfall. I also unveiled unexpected diel movements towards roosts a few hundred meters uphill from the breeding territory.
The final chapter extended the new insights on migration to another system in the southern hemisphere, where bird migration is still poorly understood. I applied the analytic framework used for Wheatears to present the full annual cycle of a small-sized intra-African migrant for the first time. Based on multi-sensor logger data, I retrieved stationary sites, flight behaviour and migration timing from five Woodland Kingfishers Halcyon senegalensis breeding in South Africa. These birds tracked rainy seasons in both hemispheres, migrating 4000 km north to the same area in South Sudan, where they found habitats similar to those at the breeding site. Migratory flights occurred at night like in Northern Wheatears but were shorter on average. Post-breeding migration was longer than pre-breeding migration. Woodland Kingfishers changed their flight behaviour above the Congo Basin, where I presumed that less suitable stopover habitat resulted in shorter stops as well as in faster, higher, and longer flights. This was a behaviour comparable to that of Wheatears crossing the Mediterranean Sea and the desert. Such findings became only recently possible for small birds by using multi-sensor loggers, which can be used to study intra-tropical and other migratory systems, promising major future advances for global migration research.
To conclude, this thesis establishes a link between the breeding ecology of the Northern Wheatear in the Alps and the other parts of its annual cycle, and further extending to another migratory system. It sheds light on the remarkable challenges faced by Wheatears, flying thousands of kilometres up to 5000 m in the sky, overcoming the heat of the desert and the dangers of the sea, all to reach our harsh mountains still blanketed in snow with freezing temperatures. Once the Wheatears complete their migration, they must then rear their broods amidst the constant threat of predation before preparing for the next journey, resulting in a truly awe-inspiring annual cycle.