THERMAL EFFECTS OF RADIATION AND WIND ON A SMALL BIRD AND IMPLICATIONS FOR MICROSITE SELECTION

The physical environmental factors (air temperature, solar radiation, wind speed) that define specific microclimates and their effects on wa ter and energy budgets of small birds are of major importance to our u nderstanding oi avian thermal biology. We examined the effects of sola r radiation, wind speed, and their interaction on metabolic rates in t he Verdin, Auriparus flaviceps. Daytime resting metabolic rates and ev aporative water loss rates as a function of air temperature, as well a s basal metabolic rate, were also measured to allow estimation of wate r- and energy Aux rates in diverse microclimates. In the absence of so lar radiation, as wind speed was increased from 0.4 to 3.0 m/s, metabo lic rate increased 14%. Exposure to simulated solar radiation signific antly reduced metabolic heal production at all wind speeds measured ex cept 3.0 m/s. Solar heat gain (SHG) was estimated for an irradiance of 1000 W/m(2), similar to that commonly observed in nature. At 0.4 m/s wind speed and 1000 W/m(2) irradiance, SHG may reduce metabolic rate b y 46%. SHG declines precipitously as wind speed is increased, and at 3 .0 m/s, metabolic rate is only reduced by 3%. Analyses of changes chan ges in thermostatic costs associated with microclimate selection in wi nter suggest that Verdins may reduce metabolic rare by as much as 50% by shifting from a shaded, windy site to one protected from the wind a nd exposed to 1000 W/m(2) solar radiation. Similar analyses for Verdin s during the summer suggest that microsite selection can result in sig nificant water savings. By remaining our of the sun and wind, Verdins can reduce their rate of evaporative water loss by at least a factor o f four, This analysis clearly demonstrates the potential importance of daytime microclimate selection to balancing water and energy budgets in small birds.