Outflying climate change Optimal timing of migratory geese breeding in a warming Arctic

Climate change is occurring at different rates along the flyways of migratory birds, with the most rapid climate warming in the Arctic. Migratory birds breeding here may be unable to advance migration timing to match this shift in environmental conditions, as they are (1) constrained in time to prepare for an earlier migration, or (2) as their wintering grounds do not provide cues that correctly predict the optimal timing of migration. The central aim of this thesis is to uncover the most important limiting factor for advancing timing of migration and reproduction for an Arctic migratory bird, the barnacle goose. By combining model predictions, data from long-term field studies and data from modern tracking-devices, it is shown that geese are initially not time-constrained to advance fuelling for migration. However, they seem to miss cues to adjust their migration departure from temperate wintering and stopover sites to changing conditions in the Arctic. Once in the Arctic, geese can advance arrival in early springs by accelerating migration speed. Nevertheless, they become time-constrained to adequately adjust reproduction timing, as they first need to refuel on the breeding grounds after a fast migration before they are able to breed. A phenological mismatch arises when geese breed too late in relation to the local peak in food quality, and their chicks suffer from reduced survival. While geese show some flexibility to adjust migration timing in the Arctic, successful adaptation to a warming Arctic may hinge on their ability to adjust migration timing in temperate regions.