Analysis of almost a hundred long distance migration paths of Italian and Swiss barn swallows (Hirundo rustica) reconstructed on the basis of light level geolocators

Barn swallow (Hirundo rustica) is one of the most iconic migratory bird, worldwide known for its fascinating seasonal journeys. Each year, every individual encompasses thousands of kilometres looking for favourable environmental conditions where it could overwinter in Africa, returning the typically at the same nest in Europe the subsequent spring for breeding. Despite the numerous attempts to track barn swallow’s migration throughout the last century (e.g. by ringing data, genetic analyses and stable isotopes), the reconstruction of the first complete paths of very few individuals (six individuals in total) has been possible just in recent years, thanks to the use of light level geolocators. These devices, used for measuring daylight throughout the whole migration of each specimen, are now a robust technology applied for tracking migratory animals, in particular those that are too small to carry satellite tags (e.g. passerine birds). In the last decade, different methods have been proposed to reconstruct migration paths from geolocators data, based on astronomical equations binding variation of solar irradiance, the hour and the date during twilight events. Here we analysed almost a hundred of light-level geolocators data of Italian and Switzerland barn swallows, tagged in the years 2010, 2011 and 2012. We used TWGeos, FLightR and trajr R packages respectively to elaborate geolocators data, to perform path estimation and to compute trajectories indices. The analysis of migration trajectories revealed that spring migration display more complex paths than autumn migration, not only in terms of inter-individual geographical variability, but also in terms of path tortuosity. We noticed that more individuals made a westward detour during their spring migration, some of them performing a journey even double compared to the great circle distance between wintering and breeding positions. A first set of statistics on trajectories is presented and studied in relation to timing of departure and arrival. Statistical analysis reveal that the year of migration has a significant effect on trajectories indices, while there is less influence from the population or sex of the individual. This could be a clue of the importance of en-route weather conditions on barn swallow migration. Our reconstruction may serve as a basis for more in depth and possibly mechanistic investigation on the movement ecology of European barn swallows.