Seasonal bird migration may provide energy benefits associated with moving to areas with less physiologically challenging climates or increased food availability, but migratory movements themselves may carry high costs. However, time-dynamic energy profiles of free-living migrants—especially small-bodied songbirds—are challenging to measure. Here we quantify energy output and thermoregulatory costs in partially migratory common blackbirds using implanted heart rate and temperature loggers paired with automated radio telemetry and energetic modelling. Our results show that blackbirds save considerable energy in preparation for migration by decreasing heart rate and body temperature 28 days before departure, potentially dwarfing the energy costs of migratory flights. Yet, in warmer wintering areas, migrants do not appear to decrease total daily energy expenditure despite a substantially reduced cost of thermoregulation. These findings indicate differential metabolic programmes across different wintering strategies despite equivalent overall energy expenditure, suggesting that the maintenance of migration is associated with differences in energy allocation rather than with total energy expenditure.

季节性鸟类迁徙可能会提供与迁移到生理气候挑战性较小或食物供应增加的地区相关的能源效益，但迁徙迁徙本身可能会带来高昂的成本。然而，自由生活的迁徙者（尤其是体型较小的鸣禽）的时间动态能量分布很难测量。在这里，我们使用植入的心率和温度记录器以及自动无线电遥测和能量建模来量化部分迁徙普通黑鸟的能量输出和体温调节成本。我们的结果表明，黑鸫通过在出发前 28 天降低心率和体温来为迁徙做准备，从而节省大量能量，这可能会使迁徙飞行的能源成本相形见绌。然而，在较温暖的越冬地区，尽管体温调节成本大大降低，但迁徙者似乎并没有减少每日总能量消耗。这些发现表明，尽管总能量消耗相同，但不同越冬策略的代谢计划不同，这表明迁移的维持与能量分配的差异有关，而不是与总能量消耗有关。