Population declines and carry over effects in sub-Saharan migrant birds

Across the globe anthropogenic changes are leading to population declines of migratory bird species. Despite songbirds being one of the most at-risk migratory groups, understanding of their lifecycle remains limited compared to non-migratory species, with knowledge of ecology and behaviour outside of the breeding period and range especially lacking. This thesis initially takes a global perspective to understand the risk of migration as a strategy. I synthesised population trend data from 94% of extant bird species to examine whether migratory species are more likely to be in decline than non migrants, and potential geographic and habitat related associations in which populations are more likely in decline. Subsequent chapters focus on the migratory European pied flycatcher Ficedula hypoleuca to explore possible causal mechanisms implicated in population declines of many Palaearctic-African migratory songbirds. I used a combination of individual based tracking, demographic and investigative methods from two European breeding populations, and fieldwork in West African non-breeding areas to achieve this. Our global analysis identifies widespread declines across all taxa, although migratory bird species are less likely to be in population decline than non-migrants, suggesting that some migrants may be more robust to the threats of environmental change. Among migrants, species distributed throughout the year in the northern hemisphere, within the Palearctic Asian-Australian flyway, or that use savanna habitat are declining more than other migrants. In the subsequent chapters, I describe the migratory behaviour of pied flycatchers across an annual cycle. I find that events and processes experienced during the non breeding season have influence on subsequent breeding timing and success, mediated by spring stopover behaviour. During potentially costly barrier crossing flights, the environmental conditions encountered influences migratory plasticity. Finally, I identify an association between habitat selection in non-breeding areas with disease prevalence, which may have long-term consequences for population dynamics. By identifying the flyways and habitats most associated with migrant decline, the global analysis has the potential to guide conservation and research priorities. Through working in both breeding and non-breeding populations of pied flycatchers, I show that the conditions experienced in the West Africa non-breeding range, and during spring migration, can influence subsequent reproductive success through multiple mechanisms. Our results advance understanding of pied flycatcher non-breeding ecology. These findings can be generalised to other migratory songbirds, in particular, the influence of environmental conditions on migratory behaviours