Wintering Snow Buntings Elevate Cold Hardiness to Extreme Levels but Show No Changes in Maintenance Costs

Limited heat tolerance in an Arctic passerine: Thermoregulatory implications for cold‐specialized birds in a rapidly warming world

“…Indeed, buntings overwinter in snowy climates before migrating north through equally harsh conditions during the spring, only to arrive on their Arctic breeding grounds when T a remains below freezing (Meltofte, 1983; Snell et al., 2018). Consequently, buntings have evolved physiological traits to withstand extreme cold (Scholander et al., 1950) and can tolerate experimental temperatures as low as −90°C (Le Pogam et al, 2020). Only later in the season, near the breeding period, do buntings regularly experience maximum temperatures above freezing (Meltofte, 1983).…”

Snow buntings preparing for migration increase muscle fiber size and myonuclear domain in parallel with a major gain in fat mass

“…non-significant 22% increase in lean dry mass between early March and early April) could thus translate into smaller changes in muscle thickness, making spring increases too subtle for detection by this method (Royer-Boutin et al 2015). Consequently, both our previous work and this study suggest that snow buntings enlarge their flight muscles in winter to cope with the cold, and maintain that muscle phenotype through migration and summer (Le Pogam et al 2020, but that a further increase in pectoralis mass likely occurs in parallel with fat accumulation in preparation for migration. A seasonal, dissection-based, body composition study would be required to test predictions fully and refine this interpretation.…”

“…Previous work in the same captive environment showed that buntings increase the thickness of their pectoralis muscles as they acclimate to winter conditions (Le Pogam et al 2020), and that these muscles remain large throughout the spring migratory transition and most of summer (Le Pogam et al 2021, Fig. 1).…”

“…During each sampling day, birds were captured with a handheld net and brought into the lab adjacent to the aviary where they were weighed (± 0.01 g). We measured the fat and lean component of body mass non‐invasively by quantitative magnetic resonance (QMR, ± 0.01 g; EchoMRI, Houston, TX, USA; Guglielmo et al 2011, see Le Pogam et al 2020, 2021 for details). We then euthanatized birds by CO 2 asphyxiation, after which we excised a sample of the left pectoralis muscle (~0.5 g) within 2 min of death, which we immediately fixed inside an avian ringer's solution containing 4% paraformaldehyde.…”

“…These birds are also known to maintain high oxygen carrying capacity and large flight muscles during this period (based on muscle thickness measured by ultrasonography, Le Pogam et al 2021). In fact, these two traits are both upregulated during the fall (Sept–Nov), maintained through the winter months and spring transition into a migratory phenotype (March, Le Pogam et al 2020) and up to the beginning of summer (April–May, Le Pogam et al 2021). Working with birds under outdoor captive conditions, we therefore expected to observe the previously reported increases in fat and body mass while we predicted pectoralis muscle mass would remain stable during that period (Fig.…”