Enriching the isotopic toolbox for migratory connectivity analysis: a new approach for migratory species breeding in remote or unexplored areas

Aim: We examined three potential enhancements of the stable isotope tech- nique for elucidating migratory connectivity in birds inhabiting poorly studied areas, illustrated for Eurasian cranes (Grus grus) that overwinter in and migrate through Israel. First, we examined the use of oxygen stable isotopes (d18O), sel- dom applied for this purpose. Second, we examined the relationship between ambient water d18O and hydrogen stable isotope (d2H) values derived from various models, to determine the geographical origins of migrants. Third, we introduced the use of probabilistic distribution modelling to refine the assign- ment to origin of migrants lacking detailed distribution maps. Location: Feather samples were collected in the Hula Valley (northern Israel) and across the species breeding range in north Eurasia. Methods: We analysed d18O and d2H in primary and secondary flight feathers using standard mass spectrometry. The maximum entropy (MAXENT) model was used to map the probability surface of potential breeding areas, as a Bayesian prior for assigning Hula Valley cranes to potential breeding grounds. Results: We found that d18O was suitable and informative. The soil water iso- scape performed better for d18O while precipitation isoscape was preferable for d2H. The MAXENT-based probability surface largely refined assignments. Overall, most (>85%) cranes were assigned to the area west of the Ural Mountains, but for two individuals, most of the assigned area (>90%) was farther east, suggest- ing, for the first time, that Eurasian cranes may undertake the North Asia–Mid- dle East (and perhaps Africa) migration flyway. Main conclusions: Our results call for broader use of d18O in migratory con- nectivity studies and for application of probabilistic distribution modelling. We also encourage investigation of factors determining d18O and d2H integration into animal tissues. The proposed framework may help improve our under- standing of migratory connectivity of species inhabiting previously unexplored areas and thus contribute to the development of efficient conservation plans.