Seasonal phenotypic flexibility in body mass, basal thermogenesis, and tissue oxidative capacity in the male Silky Starling (Sturnus sericeus)

Background Acclimatization to winter conditions is an essential prerequisite for the survival of small birds in the northern temperate zone. Changes in photoperiod, ambient temperature and food availability trigger seasonal physiological and behavioral acclimatization in many passerines. Seasonal trends in metabolic parameters are well known in avian populations from temperate environments; however, the physiological and biochemical mechanisms underlying these trends are incompletely understood. In this study, we used an integrative approach to measure variation in the thermogenic properties of the male Silky Starling (Sturnus sericeus) at different levels or organization, from the whole organism to the biochemical. We measured body mass (M_b), basal metabolic rate (BMR), energy budget, the mass of selected internal organs, state 4 respiration and cytochrome c oxidase (COX) activity in the heart, liver and muscle.

Methods Oxygen consumption was measured using an open-circuit respirometry system. The energy intake of the birds were then determined using an oxygen bomb calorimeter. Mitochondrial state 4 respiration and COX activity in heart, liver and pectoral muscle were measured with a Clark electrode.

Results The results suggest that acclimatization to winter conditions caused significant change in each of the measured variables, specifically, increases in M_b, organ mass, BMR, energy intake and cellular enzyme activity. Furthermore, BMR was positively correlated with body mass, energy intake, the mass of selected internal organs, state 4 respiration in the heart, liver and muscle, and COX activity in the heart and muscle.

Conclusions These results suggest that the male Silky Starlingos enhanced basal thermogenesis under winter conditions is achieved by making a suite of adjustments from the whole organism to the biochemical level, and provide further evidence to support the notion that small birds have high phenotypic plasticity with respect to seasonal changes.