Breeding origin and spatial distribution of migrant and resident harriers in a Mediterranean wintering area: insights from isotopic analyses, ring recoveries and species distribution modelling

Abstract

Variation in avian migratory behaviour is widespread, not only among species but also within species, and can involve shifts from sedentary at low latitudes to migratory at high latitudes. This leads to a situation whereby non-migratory populations of a species at lower latitudes are periodically joined by migratory conspecifics during winter. Determining spatio-temporal dynamics in distribution and structure of different populations is crucial to our understanding of their ecology and interactions, but it is often difficult or impossible to separate resident from migrant populations. Here, we used the winter distribution of Marsh Harrier (Circus aeruginosus) in the Iberian Peninsula (southwest Europe) to analyse the potential of spatial distribution models and stable isotope analyses of feathers (deuterium, δ2H) to evaluate differences in spatial distribution of migrant and resident raptor populations in southern Europe. Overall, ring recoveries and isotopic assignment showed that most migrant harriers wintering in the Iberian Peninsula had a central and northern latitudinal European origin. Our results revealed segregation in the winter distribution of migrant and resident Marsh Harrier populations in the Iberian Peninsula. These results have important conservation implications, suggesting that ecological conditions and potential stressors could greatly differ among resident and migrant populations in winter. Our approach provided a useful methodological procedure to evaluate the wintering spatial segregation of European populations of a raptor species and presents a new challenge to the study of their ecological consequences.