Background Understanding the decline of migratory birds is complex owing to the vast journeys they undertake annually. Individuals use multiple sites across the annual cycle, each associated with its own threats, and any single site can contribute to population decline. A holistic approach is required to understand population decline, investigating each stage of the annual cycle to identify potential causes. The Greenland White-fronted Goose Anser albifrons flavirostris (GWfG) is a long-distance migrant that has undergone a stark population decline since 1999. The population exhibits a metapopulation structure and the two largest sub-populations (Islay, Scotland and Wexford, Ireland) overwhelming contribute to the overall trends of the metapopulation and are the focus of this thesis. Approaches I reviewed spatial changes in avian migrations, particularly the causes and consequences of distributional shifts. The emphasis was on the inter-connected nature of migratory stages and how changes at one stage can have implications that cascade throughout the annual cycle. I then explored two key potential drivers of GWfG population decline; 1) drivers of low reproductive success, particular in the Wexford sub-population and, 2) non-breeding ground factors that could reduce the fitness of the Islay sub-population. To understand drivers of reproductive success I developed a method to identify incubation from biologging data and then applied it to GWfG to determine the drivers of reproductive success. This was complemented with a population-level analysis, investigating how broad-scale climatic factors influence reproductive output of both sub-populations. On Islay, the sympatric Barnacle Geese Branta leucopsis are managed to reduce the population size and I explored how shooting-related disturbance and environmental lead (Pb) from spent shot might have negative fitness implications for GWfG. We used biologging to examine how shooting impacted the movement, habitat selection and behaviour of both goose species. A variety of methods were used to quantify Pb ingestion levels including faecal sampling, x-rays and post mortems. Key findings The decline of GWfG has coincided with a weakening of the relationship between climatic factors on the breeding grounds and reproductive success. There has also been a significant decline in reproductive success for the Wexford sub-population but not the Islay sub-population. Using my methodology to classify incubations I found that the nest survival of the Wexford sub-population is lower and driven by different environmental and phenological factors compared to the Islay sub-population. On the Islay non-breeding grounds we found GWfG were infrequently disturbed by shooting and their response differed significantly compared to Barnacle Geese. GWfG showed little to no changes in daily energetic expenditure, habitat selection or behaviour in response to shooting. All methods used to quantify lead ingestion suggested that levels were low across both species and therefore it was unlikely to cause reduced survival/fitness in either species. Implications The review highlights the interconnected nature of migrations and that incorporating this into the modelling of climate-change driven range shifts could improve the accuracy of future predictions. There appear to be different drivers of reproductive output in the two main GWfG sub-populations. This will likely require bespoke conservation strategies targeted at the sub-population level to reverse declines. For the Wexford sub-population the most likely causes of low reproductive success are carry-over effects causing poor breeding condition, increases in predation or competition with the expanding Canada Geese Branta Canadensis interior population in western Greenland. For the Islay sub-population, changes in other demographic rates, e.g. survival and emigration, are likely implicated in declines and warrant further research. On the non-breeding grounds hunting of Islay Barnacle Geese does not appear to be having negative fitness impacts on the GWfG. The measures to minimise the effects of Barnacle Geese management on GWfG, appear to have been largely successful.