Food restriction decreases thermoregulation in the silky starling Sturnus sericeus (Aves: Passeriformes)

Abstract

The capacity of small birds to sustain periods of food shortage largely depends on their ability to adaptively regulate expenditure in response to reduced food supply. To investigate changes in thermoregulation during periods of food restriction, we measured changes in the body mass, basal metabolic rate (BMR), organ mass, thyroid hormone levels, and several physiological and biochemical markers, of silky starlings (Sturnus sericeus) subject to 16 days food restriction. Birds in the control group were provided with food ad libitum whereas those in the food restriction (FR) group were provided with half of the usual quantity of food. Oxygen consumption was measured using an open–circuit respirometry system. Mitochondrial state-4 respiration and cytochrome c oxidase (COX) activity in the liver and pectoral muscle were measured with a Clark electrode. The concentrations of T3 and T4 in plasma were determined by radioimmunoassay. Starlings in the FR group had lower body mass and BMR compared to the control group. Starlings in the FR group also had lower state-4 respiration and COX activity in the liver and muscle, and lower T3/T4 ratio in plasma, relative to the control group. BMR was positively correlated with body mass, state-4 respiration and COX activity, in liver and muscle tissue. These results indicate that silky starlings can decrease their energy metabolism during periods of food shortage by making a suite of adjustments from the organismal to the biochemical level. These morphological, physiological and biochemical responses could be adaptations that allow starlings to adjust to seasonal changes in food abundance, such as the typically lower food abundance over winter.