Thermal Effects of Radiation and Wind on a Small Bird and Implications for Microsite Selection

Abstract

The physical environmental factors (air temperature, solar radiation, wind speed) that define specific microclimates and their effects on water and energy budgets of small birds are of major importance to our understanding of avian thermal biology. We examined the effects of solar radiation, wind speed, and their interaction on metabolic rates in the Verdin, Auriparus flaviceps. Daytime resting metabolic rates and evaporative water loss rates as a function of air temperature, as well as basal metabolic rate, were also measured to allow estimation of water and energy flux rates in diverse microclimates. In the absence of solar radiation, as wind speed was increased from 0.4 to 3.0 m/s, metabolic rate increased 14%. Exposure to simulated solar radiation significantly reduced metabolic heat production at all wind speeds measured except 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 by 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 in thermostatic costs associated with microclimate selection in winter suggest that Verdins may reduce metabolic rate by as much as 50% by shifting from a shaded, windy site to one protected from the wind and exposed to 1000 W/m$2$ solar radiation. Similar analyses for Verdins during the summer suggest that microsite selection can result in significant water savings. By remaining out of the sun and wind, Verdins can reduce their rate of evaporative water loss by at least a factor of four. This analysis clearly demonstrates the potential importance of daytime microclimate selection to balancing water and energy budgets in small birds.

Avian thermoregulation in the heat: evaporative cooling capacity in an archetypal desert specialist, Burchell's sandgrouse (Pterocles burchelli)

“…The challenges of avoiding lethal hyperthermia are especially pronounced in birds because of their small body size, typically diurnal activity and limited use of thermally buffered microsites such as burrows; the adaptations that permit birds to occupy even the hottest deserts remain one of the most enduring fields of inquiry among ecological and evolutionary physiologists (Austin, 1976;Dawson and Bartholomew, 1968;Keast, 1960;Maclean, 1984;Williams and Tieleman, 2005). Thermoregulation under extremely hot conditions, when intense solar heat loads may result in birds experiencing operative temperatures 15°C or more above normothermic body temperature (T b ) (Bakken, 1976;Robinson et al, 1976;Wolf and Walsberg, 1996b), involves consequential trade-offs between hyperthermia avoidance via evaporative heat dissipation and dehydration avoidance through the conservation of body water. Sandgrouse (Pterocliformes) are the quintessential arid-adapted avian taxon.…”

The effect of energy reserves on social foraging: hungry sparrows scrounge more

“…All cages were located within the same room, so the noise disturbance was similar for all birds. Wind exposure significantly reduces surface body temperature and increases metabolic rate in small passerines (Bakken & Lee 1992;Wolf & Walsberg 1996;Zerba et al 1999), and has been successfully used to manipulate overnight energy expenditure (Witter et al 1994;Cuthill et al 2000). One might argue that restricted access to food could be a more effective means of reducing reserves.…”

Avian thermoregulation in the heat: scaling of heat tolerance and evaporative cooling capacity in three southern African arid-zone passerines

“…Daytime air temperatures (T a ) in many deserts routinely exceed the normothermic T b of mammals and birds (Dawson and Schmidt-Nielsen, 1964;Serventy, 1971) and even when T a remains below T b , the additional heat load associated with exposure to solar radiation can result in operative temperatures (sensu Bakken, 1976;Robinson et al, 1976) far above T b , particularly in small species (King and Farner, 1961;Wolf and Walsberg, 1996b). Birds provide some of the most striking examples of organisms that survive and breed in extremely hot, inhospitable environments (Grant, 1982;Tieleman et al, 2008).…”