Nomadic songbirds achieve rapid increases in thermogenic capacity during cold acclimation while maintaining low...

Migrating animals can track resources to maximize energy intake, but moving may also expose them to rapid changes in weather that require thermogenic defense. The Migratory Flexibility Hypothesis predicts that migrants exposed to variable conditions may need to be more flexible in thermogenic response. Here, we investigate metabolic flexibility of the red crossbill (Loxia curvirostra), an altitudinal and latitudinal nomad that experiences wide variability in climate, to determine how flexibility in migration relates to thermogenic flexibility. Two ecotypes of captive red crossbill (Type 3 and Type 5) that vary in degree of nomadism were transferred to thermoneutral (21 °C) or cold (6 °C) temperatures during their spring migratory period. Metabolic rates and baseline corticosterone levels were repeatedly measured across nine days during acclimation. Both ecotypes of red crossbill showed the fastest increase in thermogenic capacity (M_sum) yet documented in songbirds, and this change was accompanied by higher basal maintenance costs but not upregulation of baseline corticosterone, suggesting that acclimation did not induce allostatic overload. Furthermore, crossbills in thermoneutral temperatures, that were exposed to only a single 90-minute cold exposure during M_sum measurement, transiently upregulated M_sum, demonstrating high sensitivity to even brief cold exposure. Type 5 red crossbills, which frequently live at very high elevations, showed slightly steeper increases in M_sum despite being considered less nomadic. These findings indicate that metabolic flexibility may be advantageous for species that experience rapid temperature oscillations during migration, but the ability to upregulate may be most related to duration and extent of temperature extremes.