Small birds in temperate habitats must either migrate, or adjust aspects of their morphology, physiology and behavior to cope with seasonal change in temperature and photoperiod. It is, however, difficult to accurately measure how seasonal changes in temperature and photoperiod affect physiological processes such as basal metabolic rate (BMR) and metabolic activity. To address this problem, we collected data in each month of the year on body mass (M_b) and BMR, and conducted a series of experiments to determine the effect of temperature and photoperiod on M_b, BMR and physiological markers of metabolic activity, in the Eurasian Tree Sparrow (Passer montanus).

In one experiment, we measured monthly change in M_b and BMR in a captive group of birds over a year. In another experiment, we examined the effects of acclimating birds to two different temperatures, 10 and 30 ℃, and a long and a short photoperiod (16 h light:8 h dark and 8 h light:16 h dark, respectively) for 4 weeks.

We found that these treatments induced sparrows to adjust their M_b and metabolic rate processes. Acclimation to 30 ℃ for 4 weeks significantly decreased sparrows' M_b, BMR, and energy intake, including both gross energy intake and digestible energy intake, compared to birds acclimated to 10 ℃. The dry mass of the liver, kidneys and digestive tract of birds acclimated to 30 ℃ also significantly decreased, although their heart and skeletal muscle mass did not change significantly relative to those acclimated to 10 ℃. Birds acclimated to 30 ℃ also had lower mitochondrial state-4 respiration (S4R) and cytochrome c oxidase (COX) activity in their liver and skeletal muscle, compared to those acclimated to 10 ℃. Birds acclimated to the long photoperiod also had lower mitochondrial S4R and COX activity in their liver, compared to those acclimated to the short photoperiod.

These results illustrate the changes in morphology, physiology, and enzyme activity induced by seasonal change in temperature and photoperiod in a small temperate passerine. Both temperature and photoperiod probably have a strong effect on seasonal variation in metabolic heat production in small birds in temperate regions. The effect of temperature is, however, stronger than that of photoperiod.

The thermoregulatory ability of animals is strongly influenced by the temperature of their environment. Acclimation to cold requires a range of physiological and morphological adjustments. In this study, we tested the hypothesis that a small passerine, the Red-billed Leiothrix (Leiothrix lutea), can maintain homeothermy in cold conditions by adjusting the physiology and biochemistry of its tissue and organs and return to its former physiological and biochemical state when moved to a warm temperature.

Phenotypic variation in thermogenic activity of the Red-billed Leiothrixs (Leiothrix lutea) was investigated under warm (35 ℃), normal (25 ℃) or cold (15 ℃) ambient temperature conditions. Oxygen consumption was measured using an open-circuit respirometry system. Mitochondrial state-4 respiration and cytochrome-c oxidase (COX) activity in liver, kidney heart and pectoral muscle were measured with a Clark electrode.

Birds acclimated to an ambient temperature of 15 ℃ for 4 weeks significantly increased their basal metabolic rate (BMR) compared to a control group kept at 25 ℃. Birds acclimated to 35 ℃ decreased their BMR, gross energy intake (GEI) and digestible energy intake (DEI). Furthermore, birds acclimated to 15 ℃ increased state-4 respiration in their pectoral muscles and cytochrome-c oxidase (COX) activity in their liver and pectoral muscle, compared to the 25 ℃ control group. Birds acclimated to 35 ℃ also displayed lower state-4 respiration and COX activity in the liver, heart and pectoral muscles, compared to those kept at 25 ℃. There was a positive correlation between BMR and state-4 respiration, and between BMR and COX activity, in all of the above organs except the liver and heart.

Our study illustrates that the morphological, physiological, and enzymatic changes are associated with temperature acclimation in the Red-billed Leiothrix, and supports the notion that the primary means by which small birds meet the energetic challenges of cold conditions is through metabolic adjustments.

Food is an important environmental factor that affects animals' energy metabolism and food shortage has significant effects on animals' behavior, physiology and biochemistry. However, to date few studies have focused on the thermogenesis and its effects on the body condition of birds. In this study, we examined the effects of food restriction on the body mass, basal metabolic rate (BMR) and body composition, and several physiological, biochemical and molecular markers potentially related to thermogenesis, in the Chinese Bulbul (Pycnonotus sinensis).

Birds in the control group were provided with food ad libitum whereas those in the food restriction group were provided with one-half of the usual quantity of food for 12 days. 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. Avian uncoupling protein (avUCP) mRNA expression was determined in pectorals muscle with quantitative Real-time PCR.

Chinese Bulbuls in food restriction group decreased in body mass, BMR and internal organ (heart, kidneys, small intestine and total digestive tract) mass compared with the control group over the 12-day period of food restriction. Bulbuls in the food restriction group also had lower levels of state-4 respiration, COX activity in the liver and muscle, and mitochondrial avUCP gene expression in muscle compared to the control group. BMR was positively correlated with body mass, state 4 respiration in the liver and COX activity in the muscle.

Our data indicate that Chinese Bulbuls not only sustain food shortage through simple passive mechanisms, such as reducing body and organ mass and energy expenditure, but also by reducing energetic metabolism in the liver and muscle.

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.

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.

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.

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.

The capacity for thermogenesis is considered part of an animal's adaptive strategy for survival,and basal metabolic rate (BMR) is one of the fundamental physiological standards for assessing the energy cost of thermoregulation in endotherms. BMR has been shown to be a highly flexible phenotypic trait both between,and within,species,but the metabolic mechanisms involved in the regulation of BMR,which range from variation in organ mass to biochemical adjustments,remain unclear. In this study,we investigated the relationship between organ mass,biochemical markers of metabolic tissue activity,and thermogenesis,in three species of small passerines: wild Bramblings (Fringilla montifringilla),Little Buntings (Emberiza pusilla) and Eurasian Tree Sparrows (Passer montanus),caught in Wenzhou,southeastern China.

Oxygen consumption was measured using an open-circuit respirometry system. Mitochondrial state-4 respiration and cytochrome c oxidase (COX) activity in liver and pectoral muscle were measured with a Clark electrode.

Our results show that Eurasian Tree Sparrows had significantly higher BMR,digestive organ mass,mitochondrial state-4 respiration capacity and COX activity in liver and muscle,than Bramblings and Little Buntings. Furthermore,interspecific differences in BMR were strongly correlated with those indigestive tract mass,state-4 respiration and COX activity.

Our findings suggest that the digestive organ mass,state-4 respiration and COX activity play an important role in determining interspecific differences in BMR.

Acclimatization to winter conditions is an essential prerequisite for survival of small passerines of the northern temperate zone. In the present study, we measured diurnal variations in body mass, body temperature and basal metabolic rate (BMR) for seasonally acclimatized Hwameis (Garrulax canorus).

Body mass was determined with a Sartorius balance. Metabolic rates of Hwameis were measured with an open-circuit respirometry system.

Body masses varied with time of day and were higher in daytime for Hwameis in both summer and winter, and body masses in winter were higher compared to that in summer. Body temperatures of Hwameis were higher in daytime, and the summer acclimatized birds had significantly higher body temperatures compared to the winter acclimatized birds. BMRs of Hwameis were significantly higher during the daytime compared to the nighttime of the daily cycle in both summer and winter, and Hwameis in winter had significantly higher BMRs than that in summer.

This result showed that Hwameis rely mostly on metabolic capacity to maintain their body temperature in cold weathers, and Hwameis exhibited daily and seasonal flexibility in morphology and physiology which is important under changing environmental conditions.

Seasonal adjustments in body mass and energy budget are important for the survival of small birds in temperate zones. Seasonal changes in body mass, body temperature, gross energy intake (GEI), digestible energy intake (DEI), body fat content, as well as length and mass of the digestive tract, were measured in Chinese Bulbuls (Pycnonotus sinensis) caught in the wild at Wenzhou, China.

Body mass was determined with a Sartorius balance. The caloric contents of the dried food and feces were then determined using a oxygen bomb calorimeter. Total fat was extracted from the dried carcasses by ether extraction in a Soxhlet apparatus. The digestive tract of each bird was measured and weighed, and was then dried to a constant mass.

Body mass showed a significant seasonal variation and was higher in spring and winter than in summer and autumn. Body fat was higher in winter than in other seasons. GEI and DEI were significantly higher in winter. The length and mass of the digestive tract were greatest in winter and the magnitude of both these parameters was positively correlated with body mass, GEI and DEI. Small passerines typically have higher daily energy expenditure in winter, necessitating increased food consumption.

This general observation is consistent with the observed winter increase in gut volume and body mass in Chinese Bulbuls. These results suggest that Chinese Bulbuls adjust to winter conditions by increasing their body mass, body fat, GEI, DEI and digestive tract size.