THE EFFECT OF WIND AND AIR-TEMPERATURE ON METABOLISM AND EVAPORATIVE WATER-LOSS RATES OF DARK-EYED JUNCOS, JUNCO-HYEMALIS - A STANDARD OPERATIVE TEMPERATURE SCALE

Because wind significantly increases heat loss from birds, its effects must be incorporated in studies of behavior, habitat selection, and ecological energetics. We studied dark-eyed juncos (Junco hyemalis) exposed to air temperatures (T(a)) from -10-degrees-C to 40-degrees-C and wind speeds of 0.1, 0.64, 1.96, and 3 m/s. We measured O2 consumption, CO2 production, and evaporative water loss (EWL). Metabolic rate M increased with decreasing temperature and increasing wind speed for T(a) < 22.5-degrees-C. As T(a) dropped from 22.5-degrees-C to 15-degrees-C, M increased rapidly. The rate of increase slowed as T(a) dropped below 15-degrees-C. Metabolic rate was independent of T(a) and wind from 22.5-degrees-C to 37.5-degrees-C and appeared to increase at 40-degrees-C. The ratio of EWL to M increased with temperature, but the increase was less marked than that of the standard avian curve. Overall thermal conductance decreased as T(a) decreased from 37-degrees to 0-degrees-C. We divided the data set and used one part to define and the other to test an operative temperature scale (T(es)) for juncos. The T(es) scale eliminated wind effects on M, and data for all wind speeds fell on a common curve. A generalized passerine T(es) scale was somewhat less effective. The variation of M among individuals had a standard deviation of about 0.065 W. This is equal to the difference between metabolic rates in sheltered versus unsheltered roosts. We found evidence that 15%-20% of the individuals may have increased metabolic rate in response to fan noise or wind force at thermal neutral temperatures.