Differential flight strategies of Western Marsh Harrier Circus aeruginosus in relation to sex and age class during spring migration in the Central Mediterranean

Several bird species show differential migration in relation to age and/or sex classes, often associated with morphological and behavioural differences. The Western Marsh Harrier Circus aeruginosus is a partial migrant, showing a strong sexual dimorphism in size. This species migrates on a broad front, undertaking long water crossings through the Mediterranean Sea. In this study we carried out a two-year survey on the pre-breeding migration of this species at three sites located in the Central Mediterranean region: the islands of Ustica and Panarea (Tyrrhenian Sea) and the Strait of Messina (Peloritani Mountains). The aim of this study was to analyse the flight strategies of this broad front migrant in relation to wind patterns, such as in relation to different sex and age classes. Our results revealed differential flight behaviours among harriers belonging to different sex classes, with adult females less attracted to islands than adult males during a sea crossing, and adult males reaching higher altitude early in the season along a mountain chain. It is suggested that adult males, thanks to their smaller size, use to a larger extent soaring flight by exploiting even weak thermals en route. Unlike the Tyrrhenian islands, few immatures were seen passing along the Peloritani Mountains, probably because they fly at lower altitudes over mainland, passing over areas where they can eventually find prey and/or rest at stop-over site en route. In conclusion, our study shows that the location of the observation post can affect the result concerning migration survey of both sex and age classes in this species, leading to evident bias.