GEOGRAPHICAL ORIGIN OF BIRDS MIGRATING THROUGH ALPS: A STABLE ISOTOPE APPROACH (δ2H)

Every year hundreds of thousands of European birds travel after their breeding season through Europe to reach their wintering areas. Geographic barriers such as seas, deserts or high mountain chains force migrants into constrained flyways that obligate populations of different geographic origins to aggregate and follow the ecological corridors they find along their journeys (Cox 2010). Alps represent an ecological barrier to many bird species that migrate from central and northern breeding latitudes to southern wintering destinations, such as Mediterranean basin or trans-Saharan Africa. To cross such barrier, birds and especially small Passerines are obligated to follow the geographical morphology of the alpine valleys and use ecological bottle-necks such as mountain passes to change valley and prosecute their journey to south (Bruderer and Jenni 1990). The majority of small songbirds are migrating through Alps between August and November, showing two peaks of abundance, the first in August for trans-Saharan specimeasues and the second in October for intra-Palearctic species. Since 1996 ISPRA and CNI (Centro Nazionale di Inanellamento) are conducting a monitoring activity of bird migration through Alps, named ‘Progetto Alpi’ project and coordinated by MUSE of Trento and ISPRA. The project involves several ringing stations that work simultaneously between August and November from east to west across Italian Alps (Pedrini et al. 2008, 2012). The project aims to understand trends and timing of migration of several songbirds. Hence, it becomes crucial to know the geographic origin of populations affecting the Italian alpine migratory corridor. We measured hydrogen isotope ratios in feathers sampled in ‘Bocca di Caset’ ringing station (Trentino province) on two Passerines, European robin Erithacus rubecula and Pied flycatcher Ficedula hypoleuca. We used a known ‘local feather / precipitation δ2H’ regression to transform measured δ2H into values comparable to the mean-annual isoscape for δ2H in Europe (Hobson et al. 2004, Bowen et al. 2005). We than calculated the assignment using IsoMAP to produce probability surfaces of geographic assignments (Bowen et al. 2014). We refined obtained origin areas with prior information given by a recovery dataset and the known breeding range of the species.