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Resident temperate passerines adjust their phenotypes to cope with winter constraints, with peak performance in metabolic traits typically occurring during the coldest months. However, it is sparsely known whether cold-adapted northern species make similar adjustments when faced with variable seasonal environments. Life in near-constant cold could be associated with limited flexibility in traits underlying cold endurance. We investigated this by tracking individual physiological changes over five consecutive winters in snow buntings (Plectrophenax nivalis), an Arctic-breeding migratory passerine typically confronted with nearly constant cold. Buntings were held in an outdoor aviary and exposed to seasonal temperature variation typical of temperate zone climates. We measured phenotypic changes in body composition (body, fat, and lean mass, pectoralis muscle thickness), oxygen transport capacity (hematocrit), metabolic performance (basal metabolic rate [BMR] and summit metabolic rate [ M sum]), thermogenic endurance (time to reach M sum), and cold tolerance (temperature at M sum). Snow buntings showed flexibility in functions underlying thermogenic capacity and cold endurance comparable to that observed in temperate resident passerines wintering at similar latitudes. Specifically, they increased body mass (13%), fat mass (246%), hematocrit (23%), pectoralis muscle thickness (8%), and M sum (27%). We also found remarkable cold tolerance in these birds, with individuals reaching M sum in helox at temperatures equivalent to less than −90°C in air. However, in contrast with resident temperate passerines, lean mass decreased by 12%, and there was no clear increase in maintenance costs (BMR). Our results show that the flexibility of traits underlying thermal acclimatization in a cold-adapted northern species is comparable to that of temperate resident species living at lower latitudes and is therefore not limited by life in near-constant cold. [ABSTRACT FROM AUTHOR]

Cold acclimatization (phenotypic adjustments to cope with cold conditions) is an imperative requirement for birds living at high latitudes during the cold depths of winter. Despite the significant remodelling of key phenotypic traits and energetic costs associated with elevating cold endurance, winter cold acclimatization can also provide further carryover benefits to subsequent stages in species wintering, migrating and breeding in cold environments (e.g. the Arctic).We tested this beneficial carryover hypothesis using outdoor captive Arctic‐breeding snow buntings Plectrophenax nivalis, a cold specialist known for its impressive wintering thermogenic capabilities. We compared changes in phenotypic traits supporting cold acclimatization—body composition (body, fat, lean mass, pectoral muscle thickness), oxygen carrying capacity (haematocrit), thermogenic capacity and endurance (Msum, time to Msum), cold tolerance (Ta at Msum) and maintenance energy expenditure (BMR)—between the wintering, migratory and arrival/summer stages.Body mass (+31%), fat mass (+226%) and BMR (+13%) increased relative to the winter phenotype, likely to support the added costs of migration—that is the migratory upregulation hypothesis. In contrast, lean mass, pectoral muscle thickness, haematocrit and thermogenic capacity remained high and stable at winter level across stages in support of the thermal carryover hypothesis. The maintenance of these traits likely offers spare capacity for unpredictable cold environments expected during migration and breeding in the Arctic.Our results thus suggest that birds can extend the long‐term advantages of winter phenotypic adjustments through additional benefits to thermogenic capacity during subsequent life‐history stages. These benefits likely make it possible for Arctic‐breeding birds to maximize success across diverse life‐history stages in the face of extreme cold conditions. [ABSTRACT FROM AUTHOR]

Phenotypic flexibility has received considerable attention in the last decade; however, whereas many studies have reported amplitude of variation in phenotypic traits, much less attention has focused on the rate at which traits can adjust in response to sudden changes in the environment. We investigated whole animal and muscle phenotypic changes occurring in black-capped chickadees (Poecile atricapillus) acclimated to cold (−5°C) and warm (20°C) temperatures in the first 3 h following a 15°C temperature drop (over 3 h). Before the temperature change, cold-acclimated birds were consuming 95% more food, were carrying twice as much body fat, and had 23% larger pectoralis muscle fiber diameters than individuals kept at 20°C. In the 3 h following the temperature drop, these same birds altered their pectoralis muscle ultrastructure by increasing the number of capillaries per fiber area and the number of nuclei per millimeter of fiber by 22%, consequently leading to a 22% decrease in myonuclear domain (amount of cytoplasm serviced per nucleus), whereas no such changes were observed in the warm-acclimated birds. To our knowledge, this is the first demonstration of such a rapid adjustment in muscle fiber ultrastructure in vertebrates. These results support the hypothesis that chickadees maintaining a cold phenotype are better prepared than warm-phenotype individuals to respond to a sudden decline in temperature, such as what may be experienced in their natural wintering environment. [ABSTRACT FROM AUTHOR]