Individual improvements and selective mortality shape lifelong migratory performance

Abstract

A cross-sectional study of migrating raptors aged from 1 to 27 years old shows that migratory performance gradually improves with age and is driven both by selective mortality and individual improvement, with younger birds leaving progressively earlier as they age and becoming more proficient at coping with adverse environmental conditions, such as unfavourable winds. Migration, an important event in the life of many living organisms, can now be studied in minute detail using the latest remote sensing technologies. Here Fabrizio Sergio et al. use miniaturized GPS-satellite transmitters to tackle the little understood question of whether individual animals get better at migration as they get older. Data from 92 individual black kites aged between 1 and 27 years, covering more than 300 migrations between Africa and Europe, show that a bird's ability to migrate improves gradually with age. Improvement is driven by both selective mortality and individual improvement: younger birds left late and flew faster, whereas older ones left earlier and took their time, using amassed experience of wind speed and direction to ease the journey. Billions of organisms, from bacteria to humans, migrate each year1 and research on their migration biology is expanding rapidly through ever more sophisticated remote sensing technologies2,3,4. However, little is known about how migratory performance develops through life for any organism. To date, age variation has been almost systematically simplified into a dichotomous comparison between recently born juveniles at their first migration versus adults of unknown age5,6,7. These comparisons have regularly highlighted better migratory performance by adults compared with juveniles6, but it is unknown whether such variation is gradual or abrupt and whether it is driven by improvements within the individual, by selective mortality of poor performers, or both. Here we exploit the opportunity offered by long-term monitoring of individuals through Global Positioning System (GPS) satellite tracking to combine within-individual and cross-sectional data on 364 migration episodes from 92 individuals of a raptorial bird, aged 1–27 years old. We show that the development of migratory behaviour follows a consistent trajectory, more gradual and prolonged than previously appreciated, and that this is promoted by both individual improvements and selective mortality, mainly operating in early life and during the pre-breeding migration. Individuals of different age used different travelling tactics and varied in their ability to exploit tailwinds or to cope with wind drift. All individuals seemed aligned along a race with their contemporary peers, whose outcome was largely determined by the ability to depart early, affecting their subsequent recruitment, reproduction and survival. Understanding how climate change and human action can affect the migration of younger animals may be the key to managing and forecasting the declines of many threatened migrants.