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 technique 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 (δ¹⁸O), seldom applied for this purpose. Second, we examined the relationship between ambient water δ¹⁸O and hydrogen stable isotope (δ²H) values derived from various models, to determine the geographical origins of migrants. Third, we introduced the use of probabilistic distribution modelling to refine the assignment 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 δ¹⁸O and δ²H 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 δ¹⁸O was suitable and informative. The soil water isoscape performed better for δ¹⁸O while precipitation isoscape was preferable for δ²H. 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, suggesting, for the first time, that Eurasian cranes may undertake the North Asia-Middle East (and perhaps Africa) migration flyway. Main conclusions: Our results call for broader use of δ¹⁸O in migratory connectivity studies and for application of probabilistic distribution modelling. We also encourage investigation of factors determining δ¹⁸O and δ²H integration into animal tissues. The proposed framework may help improve our understanding of migratory connectivity of species inhabiting previously unexplored areas and thus contribute to the development of efficient conservation plans.