Migratory behaviour and ecology of a trans-Saharan migrant raptor, the osprey Pandion haliaetus

The seasonal migration of birds is one of the great phenomena of the natural world and satellite tracking provides a valuable means to analyse the behavioural and environmental factors that influence it. In this study satellite telemetry was used to track ospreys Pandion haliaetus during migration between the United Kingdom and West Africa. Autumn migrations were faster than those in spring, with more favourable meteorological conditions resulting in ospreys requiring fewer travelling days to reach their destination. They also incorporated time-minimisation techniques during southward journeys, indicating that selection-pressure influences migration speed in autumn as well as during spring migrations. The reclamation of winter territories is likely the key behavioural driver during autumn, particularly as later-departing individuals migrated faster. High resolution GSM-GPS transmitters provided new insights into the ability of ospreys to adapt flight method to environmental conditions, with tagged individuals exploiting thermal updrafts when available, but swapping to energy-demanding flapping flight when necessary. Very long ocean crossings, particularly across the Bay of Biscay, were regularly undertaken in autumn, when tailwinds aided progress. These flights were predominantly undertaken by flapping, but ospreys sometimes exploited weak thermals and elements of the wind to achieve soaring-gliding flight over the sea, the first time such behaviour has been documented. Individuals also regulated both flapping and gliding airspeed in response to changing wind conditions. Juvenile ospreys showed clear individual variation in the timing and speed of migration. Migration routes during the first migration were profoundly influenced by weather conditions, with wind drift resulting in very long flights across the ocean. It was also notable that ospreys with the longest post-fledging phase migrated fastest. Juveniles generally exhibited energy-minimisation techniques during migration, indicating that they were less time-constrained than adults. This may be particularly important given that individuals are likely to gain fitness advantages by arriving at the wintering grounds in good condition.