Understanding declines in the population size of migratory birds

Mondain-Monval, Thomas and Sharp, Stuart and MacColl, Andrew (2020) Understanding declines in the population size of migratory birds. PhD thesis, Lancaster University.

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Abstract

The common sandpiper is an Afro-Palearctic migrant that is declining across Europe. Studies have suggested that environmental conditions during winter are likely to be important determinants of their population trends, but these trends differ between breeding regions suggesting that wintering conditions are not the only important factor. We investigated the factors affecting common sandpipers during each of their lifecycle stages to obtain a complete overview of the factors affecting their status. During the breeding season, we found that disturbance appears to have an important effect on the probability of a nest hatching successfully. Further, we found that heavy rainfall in the week after hatching reduced the probability of chicks fledging, presumably through the influence of poor weather on thermoregulation and foraging. Common sandpipers are known to be associated with areas of high water quality. In winter, common sandpipers were more likely to be found in areas of low salinity and high pH, and had higher foraging success in these areas, suggesting that water chemistry might be an important influence on their habitat selection. Further, we showed that common sandpipers are territorial in winter, which has previously only been suggested based on anecdotal evidence. Studies of many other species have shown that the conditions during migration are likely to be a key driver of population trends. We investigated the migration of individuals across multiple populations using geolocators, tagging common sandpipers in England and Senegal, and combining these data with published data from individuals tagged in Scotland. We revealed that there is a large amount of overlap in the non-breeding distributions of individuals from these populations. Also, we showed that birds appear to use wind to facilitate their migration in autumn, but actively fly against prevailing conditions in spring. Finally, we investigated changes in the timing of wading bird migration at a flyway scale using the eBird citizen science dataset. We showed that, contrary to the findings of many studies, the timing of migration seems to be becoming later at a flyway scale and suggest range shifts as the primary driver. Understanding the influence of conditions at each lifecycle stage is paramount for determining the drivers of declining migratory bird populations. Further, investigating the mechanisms driving population trends in individual species will help us to understand the patterns we see at larger spatial scales.