Differential migration and body condition in Northern Wheatears (Oenanthe oenanthe) at a Mediterranean spring stopover site

Abstract

In many species of migratory birds, individuals of different populations, sexes and age classes migrate at different times and manage their energy reserves according to their specific migratory strategies. In this study, we analyzed the migratory patterns related to geographic provenance, sex and age in the Northern Wheatear Oenanthe oenanthe, a long-distance migratory passerine, during one spring season on the island of Ventotene (Italy), a Mediterranean stopover site. Individuals of different sex and age did not differ significantly in their average date of passage, but birds with longer and more pointed wings passed through later in the season than birds with shorter wings. Morphometric measurements combined with isotopic analysis revealed that late-arriving birds probably belonged to more distant breeding populations and this later date of passage probably mirrors the delayed arrival observed at their northern breeding grounds. Adult birds arrived in better condition than second-year birds, possibly as a result of better energy management. Birds passing through later in the season were also in better condition, which might be explained by their wing morphology favoring endurance flights and thus improving energetic efficiency when crossing the Mediterranean. Birds which migrate longer distances might also carry larger fuel loads to safely complete their journey. This intra-specific study shows that the birds’ organization of their migration schedule is population-specific in correspondence to the temporal requirements which depend on the different environmental conditions on the breeding grounds.