Graduate Thesis Or Dissertation

Seasonal thermoregulation in the dark-eyed junco (Passeriformes: Junco hyemalis)

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  • Winters in north-temperate climates impose high thermogenic demands upon small birds which are met by seasonal acclimatization. This thesis investigates the extent and mechanisms of seasonal acclimatization in darkeyed juncos (Junco hyemalis) inhabiting western Oregon. Although insulation is significantly increased in winter, acclimatization is primarily metabolic. Increased metabolism is required to offset heat loss even at moderate temperatures at both seasons. At average daily temperatures, thermoregulation in January requires 1.8 times the energy expended for thermoregulation in July and August. Standard metabolic rate (SMR) is significantly elevated in winter (3.45 mL0₂[gXhr]⁻¹) compared to summer (3.16 mL0₂[gXhr]⁻¹) , suggesting elevated metabolic capacities. Furthermore, winter birds, but not summer birds, were capable of modifying thermal conductance at low temperatures, allowing additional heat conservation. Helium/oxygen cold stress demonstrated that cold tolerance was improved in winter juncos relative to summer birds. Maximal thermogenic capacity also increased significantly in winter birds (7.39 mLO₂ /min to 5.78 m10₂/min). These values exceed SMR by 7.2 times in winter and 6.6 times in summer. Oxygen dissociation curves were generated on whole blood by saponin/potassium ferricyanide dissociation and were similar to those for other passerines. The curves did not vary significantly between summer and winter (P₅₀ = 54 torr). Hematocrit and oxygen carrying capacity were significantly increased in winter. Apparently increased oxygen demands in winter juncos are met, in part, by increased oxygen carrying capacity, but not by decreased oxygen affinity. Plasma and tissue metabolites were assayed after differing levels of cold stress to analyze seasonal variation in carbohydrate and lipid metabolism. Pectoralis muscle glycogen was significantly greater after severe cold stress in winter, although thermoneutral levels did not vary significantly. Furthermore, plasma levels of free fatty acids were significantly increased and plasma glucose was significantly decreased under severe cold. These data suggest reduced reliance upon carbohydrate for shivering thermogenesis in winter juncos and subsequent preservation of muscle glycogen stores. This may contribute to enhanced shivering endurance and cold tolerance in winter birds.
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