Energetics and Oxidative Status: Seasonal Variation in Blood Oxidative Stress Metrics in Four Species of Small Birds from a Cold Winter Climate.

Birds that overwinter in temperate regions must be physiologically flexible to face the demands of living in a thermally fluctuating environment. Much of the previous literature on this topic focuses on whole-animal metabolic rates and corresponding cellular and molecular mechanisms that enable these birds to withstand the demands of changing environmental conditions. Basal and maximal shivering metabolic rates, as well as daily energy expenditure, typically increase in winter for small birds overwintering in cold climates, which might increase the production of reactive oxygen species (ROS) within mitochondria as a natural byproduct of aerobic metabolism. In this study, we measured summer to winter differences in oxidative balance in four species of resident passerine birds. Blood samples were taken from field-collected American goldfinch (, black-capped chickadee (), house finch (), and house sparrow () during the summer and winter of 2023-2024 in South Dakota, USA. We determined plasma total antioxidant capacity and lipid oxidative damage, and red blood cell activities of three antioxidant enzymes: catalase (CAT), glutathione peroxidase (GPx), and superoxide dismutase (SOD). Lipid oxidative damage was significantly lower in winter for three of four species, and total antioxidant capacity for all species was significantly lower in winter compared with summer. Across all species, CAT activity was significantly higher in summer than in winter. In contrast, SOD activity was significantly higher in winter than in summer for all species. We also found species-level differences across the two seasons. These data suggest that the higher thermoregulatory costs in winter do not result in consistently elevated oxidative damage or antioxidant capacities relative to summer in small resident birds in cold climates, despite previously demonstrated winter increases in metabolic rates and energy expenditure. Such a result might occur as a function of either a reduction in dietary antioxidants and/or uncoupling of ROS production and metabolism in winter relative to summer or may be related to oxidative costs associated with reproduction.