Mitochondrial remodelling supports migration in white-crowned sparrows ( <i>Zonotrichia leucophrys</i> )

The migratory movements undertaken by birds are among the most energetically demanding behaviours observed in nature. Mitochondria are the source of aerobic energy production on which migration depends, but a key component of mitochondrial function, mitochondrial remodelling, has not been investigated in the context of bird migration. We measured markers of mitochondrial remodelling in the skeletal muscles of the Gambel's (migratory) and Nuttall's (non-migratory) white-crowned sparrows within and outside migratory periods. Gambel's were collected in (i) a non-migration period (baseline), (ii) preparation to depart for spring migration (pre-migration) and (iii) active autumn migration (mid-migration). Nuttall's were collected at timepoints corresponding to baseline and mid-migration in Gambel's. Across all sampling periods, we found that migratory birds had greater mitochondrial remodelling compared with non-migratory birds. Furthermore, birds from the migratory population also displayed flexibility, increasing several markers of mitochondrial remodelling (e.g. NRF1, OPA1 and Drp1) pre- and during migration. Further, the greater levels of mitochondrial remodelling and its upregulation during migration were specific to the pectoralis muscle used in flapping flight. Our study is the first to show that mitochondrial remodelling supports migration in Gambel's white-crowned sparrows, indicating a highly specific and efficient phenotype supporting the increased energetic demands of migration.