Age-specific migration timing affects the energy seascape over an ecological barrier

Background: Soaring birds extract energy from the atmosphere to achieve energetically low-cost movement. Species that fly over ecological barriers, such as open seas, need to adjust the spatio-temporal patterns of their passage to extract the highest possible energy from the atmosphere over the barrier. We introduce the concept of energy seascapes in migration of terrestrial birds and test its application in autumn migration of European honey buzzards Pernis apivorus over the Mediterranean Sea. Methods: We used bio-logging data to identify the most important environmental variables affecting the sea-crossing length in adult and juvenile honey buzzards. Then, considering this variable as a proxy for available energy over the sea, using forty years of temperature data, we constructed energy seascapes for two periods in autumn, corresponding to adult and juvenile migration periods. Results: We found temperature gradient over the sea to be an important determinant of sea-crossing length and therefore constructed energy seascapes based on this variable. Our results revealed a temporal variation in the energy seascapes, with higher energy available over the sea when juveniles migrate. Conclusions: This finding helps to explain how juvenile honey buzzards achieve a high survival rate on their first autumn migration, even though they make much longer flights over the sea than adults. Our approach to constructing energy seascapes was successful in showing that dynamics of energy availability over ecological barriers affect the strategy that soaring birds adopt to tackle them.