SEASONAL THERMAL ACCLIMATIZATION IN SEDENTARY AND ACTIVE PIGEONS

Abstract

Thermal and metabolic responses of pigeons maintained outdoors throughout the year in small cages (sedentary birds) or in large aviaries (active birds) were measured at three ambient temperatures (20, -10, -40 ° C) in midwinter and midsummer. In winter, all birds maintained a significantly lower heat production (measured as oxygen consumption and shivering), body temperature, and conductance than in summer at the two lower test temperatures. All birds maintained homeothermic body temperatures down to -40 ° C. Activity level had no effects on these thermoregulatory variables. Body mass was significantly higher in winter birds, and pectoral mass was correspondingly increased in winter. An additional specific increase in pectoral muscle mass was seen in sedentary birds. Muscle cytochrome oxidase levels were significantly higher in active birds, especially during winter. In winter, the birds had a higher plasma triiodothyronine level than in summer, and lowest values were found in active summer birds. We conclude that seasonal thermal acclimation in pigeons takes place to a great extent by insulative adjustments, which is reflected in the conductance values. Activity-induced changes in the oxidative capacity of the main heat-producing organ, the pectoral muscle, or plasma thyroid hormone levels do not influence thermal responses. Thus, there seems to be little interaction between activity-and acclimatization-induced metabolic responses in pigeons.