Arctic-nesting birds are declining and at risk due to predicted ecological effects of climate change at high latitudes. Tracking the migration of these small migratory birds can provide insight into the factors driving their spatial and behavioural patterns, but is a challenge due to their small body size, long-distance migrations and remote breeding locations. We integrated spatial and behavioural information from multiple concurrent sources (banding, stable isotopes and geolocators) from the sexually dimorphic snow bunting ( Plectrophenax nivalis ) and found strong evidence for an east-west parallel migratory system in North America. Our results also suggest that observed wintering sex-segregation is driven by the improved cold tolerance of larger males, and by selection for early arrival of males to the breeding grounds. These results improve the abilities of population models to predict and respond to declining population trends, and identifies the different selective forces that may constraint individuals in their to response to future environmental challenges