Data from: Wintering closer to breeding grounds comes at a cost in an Arctic-specialized songbird, <em>Plectrophenax nivalis</em> (Snow Bunting)

Wintering at high latitudes may offer Arctic-breeding migratory birds advantages such as shorter migration distances and earlier arrival at breeding sites, potentially improving reproductive success. However, wintering at northern sites may also come at a cost as birds can experience colder and harsher conditions than individuals wintering further south, consequently requiring physiological adjustments for life in the cold. We compared the physiological costs of wintering at different latitudes in Plectrophenax nivalis (Snow Buntings) from an Eastern Canadian population during the peak of winter. We hypothesized that individuals wintering at higher (i.e., colder) latitudes would be structurally larger, have better thermal insulation, maintain thicker pectoralis muscles and more lean tissues, carry larger lipid reserves, and experience greater physiological maintenance costs compared to individuals wintering at lower (i.e., warmer) latitudes. Contrary to our expectation, structural size did not differ between wintering sites. However, northern birds were 8% heavier and carried 32% more lipid reserves, suggesting increased energy storage for cold endurance. These birds also had 4% more lean mass and maintained slightly thicker pectoralis muscles, consistent with greater shivering thermogenesis capacity. Notably, basal metabolic rate (BMR), corrected for lean mass, was 6% higher in northern birds, indicating elevated physiological maintenance costs in colder environments. These findings therefore show that P. nivalis must pay a significant and measurable cost when wintering in colder environments to benefit from the potential fitness advantage of a shorter migration distance to their Arctic breeding grounds. Our study illustrates the physiological trade-off migratory birds face when selecting wintering locations and underscores the importance of considering both ecological benefits and metabolic costs when studying avian life-history strategies under variable climatic conditions.