Climate is warming faster in the Arctic than elsewhere in the northern hemisphere. As a result, migratory Arctic-nesting birds may arrive too late to benefit from the Arctic growth pulse. On the other hand, they may be able to invade areas that are currently becoming snow-free too late. In order to understand and predict the consequences for these birds, we therefore have to study both these processes. Pink-footed geese (Anser brachyrhynchus) traditionally nest in western Svalbard, but are now invading eastern Svalbard where the breeding season starts later. We will tag female geese in western and eastern Svalbard with GPS collars that will yield year-round information about their location and behaviour (through calibrated accelerometry). We aim to test whether geese nesting in eastern Svalbard arrive and commence incubation later, but relatively earlier after the local snow melt than those nesting in western Svalbard, whether they are genetically differentiated, and whether geese nesting in western and eastern Svalbard use different migration schemes or routes already before reaching the breeding grounds. We will extend an existing agent-based model to simulate the migration of the geese to the breeding grounds. The tracks of the geese will be used to test the model. We will develop this further into a year-round model, enabling us to predict future population size, spread and migratory behaviour of the geese in response to changes in climate, hunting and farming, and abundance of competing goose species.