The impacts of climate change on long-distance migratory birds

Abstract

Biodiversity is declining globally, and, increasingly, these declines are attributed to the impacts of climate change. Migratory species are particularly vulnerable to these impacts due to an increased likelihood of encountering environmental change. Potentially as a result, migrants have experienced rapid, recent population declines, with long-distance migratory birds having declined at faster rates than both residents and even their short- distance migratory counterparts. Given the prevalence of long-distance migratory behaviour across birds, such declines are likely to have contributed significantly to the overall loss of biodiversity and its associated ecosystem services. However, our understanding of the past, present, and future impacts of climate change on these species and, therefore, our ability to implement measures to prevent further declines remains limited. In this thesis, I use of a variety of datasets and statistical techniques to fill in a number of these knowledge gaps, identifying the impacts of climate change on long-distance migratory birds in three main areas: (1) migratory phenology, (2) population trends, and (3) migratory journeys. First, I make use of citizen science and bird observatory records to show that pre-breeding departure from the non-breeding grounds has advanced over the last 60 years, whilst post-breeding migration has been delayed. I highlight the real possibility that continuation of these trends could result in long-distance migrants “short-stopping” and adopting a short-distance migratory strategy. Next, I demonstrate, for the first time, that trends in and the drivers of the population trends of migratory birds are similar across species breeding in North America and Europe. On the breeding grounds, climate appears to be more important than land- use, whereas the converse is true on the non-breeding grounds. I then show that the climate-driven poleward shifts in species’ ranges that are predicted for the end of the century would see the breeding and non-breeding ranges of the world’s long-distance, but not short-distance, migratory birds shift in opposite directions. As a result, individuals will be required to migrate further, making additional stopovers to refuel and, therefore, spending longer on migration overall. Finally, I use reconstructions of pre-historic climate to demonstrate that, whilst migration distances were likely to have previously been significantly shorter, long-distance migration was as prevalent a phenomenon at the last glacial maximum as it is today. Together, my results show that, whilst species show some adaptive capacity to climate change, long-distance migrants are unlikely to be able to cope with further changes in climate and, without help, will suffer further declines. Furthermore, we still lack vital data for the study of migratory birds, which precludes a complete understanding of their population trends. However, given the spatial consistency of my findings, future conservation planning that accounts for the increased vulnerability of migratory species can likely be implemented in spatially disparate locations with similar success.