Interspecific variation of thermoregulation between small migratory and resident passerines in Wenzhou

Physiological adaptation arises from several fundamental sources of phenotypic variation. Most analyses of metabolic adaptation in birds have focused on the basal metabolic rate (BMR), the lower limit of avian metabolic heat production. In this study, we investigated thermoregulation in three passerine species; the yellow-billed grosbeak Eophona migratoria, white-rumped munia Lonchura striata and black-throated bushtit Aegithalos concinnus, in Wenzhou, China. Metabolic rate was measured using the closed-circuit respirometer containing 3.5 L animal chambers. Body temperature (T_b) was measured during metabolic measurements using a lubricated thermocouple. The minimum thermal conductance of these species was calculated by measuring their T_b and metabolic rates. The yellow-billed grosbeak remained largely normothermic, and the white-rumped munia and black-throated bushtit exhibited variable T_b at ambient temperatures (T_a). Mean metabolic rates within thermal neutral zone were 2.48±0.09 O₂(mL)/g/h for yellow-billed grosbeaks, 3.44±0.16 O₂(mL)/g/h for white-rumped munias, and 3.55±0.20 O₂(mL)/g/h for black-throated bushtits, respectively. Minimum thermal conductance of yellow-billed grosbeak, white-rumped munia and black-throated bushtit were 0.13±0.00, 0.36±0.01, and 0.37±0.01 O₂(mL)/g/h/℃, respectively. The ecophysiological characteristics of these species were:(1) the yellowbilled grosbeak had relatively high T_b and BMR, a low lower critical temperature and thermal conductance, and a metabolic rate that was relatively insensitive to variation in T_a; all of which are typical of cold adapted species and explain its broader geographic distribution; (2) the white-rumped munia and blackthroated bushtit had high thermal conductance, lower critical temperature, and relatively low BMR, all which are adapted to warm environments where there is little selection pressure for metabolic thermogenesis. Taken together, these data illustrate small migratory and resident passerines that exhibit the different characteristics of thermoregulation.