In most passerine birds, several body composition traits (e.g., body mass, fat reserves, muscles mass) decrease during breeding. However, a recent study on outdoor captive snow buntings (Plectrophenax nivalis) demonstrated a carry-over effect of winter cold acclimatization during the spring and part of summer, where most traits were maintained at winter level despite facing summer temperatures much warmer than on the breeding grounds. This suggest that snow bunting likely (1) maintain high cold endurance during migration and may (2) face a double physiological costs during breeding, having to support a winter phenotype while upregulating other traits for reproduction. This study aimed at testing these hypotheses. Between 2016 and 2019, we compared body composition (body mass, fat reserve and pectoral muscle thickness) and metabolic performance (thermogenic capacity and physiological maintenance costs) of birds captured on their wintering grounds (Rimouski, QC) and before breeding in the High Arctic (Alert, NU). We then studied how these traits changed from pre-breeding to chick provisioning in reproductive buntings at Alert. Our results confirm that buntings maintain a high thermogenic capacity and associated traits between winter and pre-breeding period, indicating that they likely maintain winter cold endurance during migration. While body mass, fat reserves and hematocrit decreased significantly through breeding stages, pectoral muscle thickness and thermogenic capacity seemed to decrease only from a certain temperature set point (~1.5°C) each year. These latter results suggest that buntings may be forced to maintain winter-level cold endurance late into the summer on colder years, independently of their breeding stages, which could imply additional physiological costs potentially affecting reproduction.