The study looked at wild red‑billed leiothrix birds and found they increase body weight, basal metabolic rate and use more fat in winter compared to summer.

Phenotypic flexibility enables organisms like birds to adapt to environmental changes, particularly through physiological adjustments such as basal metabolism in response to temperature fluctuations. We have previously reported that red-billed leiothrix (Leiothrix lutea) increases basal metabolism when acclimated to cold temperatures in the lab. This study investigated whether similar physiological changes also occur in response to seasonal variations in the wild. Red-billed leiothrixes were captured in different seasons in Wenzhou and subjected to experiments that measured body mass, metabolic parameters such as basal metabolic rate (BMR), cytochrome c oxidase (COX), substrates of metabolism such as serum glucose, hepatic glycogen, and muscle glycogen, triglyceride (TG), free fatty acid (FFA), enzymes such as carnitine palmitoyl transferase-1 (CPT-1), β-hydroxyacyl Co-A dehydrogenase (HOAD), and citrate synthase (CS), and thyroid hormones (T₄, thyroxine, and T₃, triiodothyronine). In winter, the birds showed significant increases in body mass, BMR, and COX activity in the liver and kidney compared to summer. Additionally, serum TG, concentration of CPT-1 and HOAD in the liver, serum glucose, and muscle glycogen were also elevated. These results were consistent with enhanced metabolism and higher consumption of lipids by red-billed leiothrixes during winter compared with other seasons. Therefore, red-billed leiothrixes show the same pattern of phenotypic flexibility as thermally acclimated birds in the lab. This study provides valuable data for understanding basal metabolism in small birds in response to seasonal changes.