Passerines that overwinter in temperate climates undergo seasonal acclimatization that is characterized by metabolic adjustments that may include increased basal metabolic rate (BMR) and cold-induced summit metabolism (M(sum)) in winter relative to summer. Metabolic changes must be supported by equivalent changes in oxygen transport. While much is known about the morphology of the avian respiratory system, little is known about respiratory function under extreme cold stress. We examined seasonal variation in BMR, M(sum), and ventilation in seasonally acclimatized house sparrows from Wisconsin. BMR and M(sum) increased significantly in winter compared with summer. In winter, BMR increased 64%, and M(sum) increased 29% over summer values. The 64% increase in winter BMR is the highest recorded for birds. Metabolic expansibility (M(sum)/BMR) was 9.0 in summer and 6.9 in winter birds. The metabolic expansibility of 9.0 in summer is the highest yet recorded for birds. Ventilatory accommodation under helox cold stress was due to changes in breathing frequency (f), tidal volume, and oxygen extraction efficiency in both seasons. However, the only significant difference between summer and winter ventilation measures in helox cold stress was f. Mean f in helox cold stress for winter birds was 1.23 times summer values.

Passerines that are year-round residents in temperate climates undergo seasonal acclimatization that facilitates maintenance of thermoregulatory homeostasis. These changes in thermoregulatory metabolism must be supported by equivalent changes in oxygen transport. We measured the effects of ambient temperature and time of day on metabolism and ventilation in House Sparrows (Passer domesticus) in summer and winter. House Sparrows were exposed to ambient temperatures (Ta) ranging from −12°C to 15.5°C in summer and 17.5°C in winter. Open-circuit respirometry was used to measure oxygen consumption (V̇O2) and ventilation was recorded using whole-body plethysmography. In both summer and winter, V̇O2 increased with decreasing Ta. V̇O2 was significantly higher during the active phase compared to the resting phase of the daily cycle in both summer and winter. Thermal conductance was significantly lower in nighttime birds compared to daytime birds. With increased oxygen demands, minute volume (V̇I) increased in both summer and winter. In summer, only respiratory frequency (f) was significantly affected by time of day. In winter, active-phase birds had significantly higher respiratory frequency and minute volume (V̇I) than rest-phase birds. Winter birds in their rest phase had significantly higher oxygen extraction efficiency (EO2) than active-phase birds. Winter birds at rest phase also had significantly higher EO2 than summer birds. Variación Estacional y Diaria en el Metabolismo y la Ventilación en Passer domesticus Resumen. Las aves paserinas que residen a lo largo del año en climas de la zona templada pasan por un proceso de aclimatación estacional que facilita el mantenimiento de su homeostasis termorregulatoria. Estos cambios en el metabolismo termorregulatorio deben estar apoyados por cambios equivalentes en el transporte de oxígeno. En este estudio medimos los efectos de la temperatura del ambiente y la hora del día sobre el metabolismo y la ventilación en Passer domesticus en verano e invierno. Las aves fueron expuestas a temperaturas ambiente (Ta) en un rango de −12°C a 15.5°C en el verano, llegando a 17.5°C en el invierno. Empleamos respirometría de circuito abierto para medir el consumo de oxígeno (V̇O2) y registramos la ventilación usando pletismografía de cuerpo entero. Tanto en el verano como en el invierno, V̇O2 aumentó con disminuciones en Ta. V̇O2 fue significativamente mayor durante la fase activa en comparación con la fase de descanso del ciclo diario, tanto en verano como en invierno. La conductancia térmica fue significativamente menor en aves estudiadas en la noche que en aves estudiadas durante el día. Con incrementos en la demanda de oxígeno, el volumen minuto (V̇I) aumentó en verano y en invierno. En verano, sólo la frecuencia respiratoria (f) fue afectada por la hora del día. En invierno, la frecuencia respiratoria y el volumen minuto (V̇I) fueron significativamente mayores en las aves en fase activa que en las aves en fase de descanso. Las aves de invierno en su fase de descanso presentaron una eficiencia de extracción de oxígeno (EO2) siginificativamente mayor que la de las aves en fase activa. La EO2 de las aves de invierno en fase de descanso también fue significativamente mayor que la de las aves de verano.

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