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But birds also show a number of attributes that potentially favor their residence in these environments. First, although their mass-specific rates of water and energy usage are relatively large, their overall resource requirements are modest because of their small body size. Thus, they can potentially inhabit regions where food and water resources are in short supply. Birds are also highly mobile and can travel long distances in search of resources in very short periods of time. As a consequence some species are able to take advantage of temporally or spatially localized resources over broad areas. In addition, the body temperatures of birds are on average 3 to 4°C higher than those of mammals. Birds also readily allow their body temperature to increase an additional 2 to 4°C in response to heat or water stress and exercise (Webster 1991). These higher body temperatures provide more favorable conditions for heat loss to the environment by mechanisms such as radiation, conduction or convection. Radiative, conductive and convective heat transfer mechanisms rely on temperature gradients to drive heat flow and thus help conserve precious water resources.
When the temperature of the physical environment approaches or exceeds avian body temperature, the thermal gradient for passive heat transfer (radiation, conduction and convection) is diminished or even reversed and evaporative water loss becomes an essential means of heat transfer. During these periods, both endogenous heat loads (metabolic heat) and exogenous heat loads (heat absorbed from the environment) must be lost through evaporation. Birds resident to sub-tropical deserts during the summer are routinely exposed to these conditions (Walsberg 1994). Air temperatures during the summer in the Sonoran Desert of Arizona, USA, may commonly exceed 42°C for 8 hours or more of each day and occasionally can even exceed 50°C (Fig. 1) (Azmet 1990). Increases in environmental temperatures above avian body temperature leads to dramatic increases in evaporative water loss. For example, in Verdins (Auriparus flaviceps), which are small (ca. 6.5 g) year-round residents of the Sonoran Desert, total evaporative water loss increases seven-fold between 38 and 48°C under resting conditions (Fig. 2) (Wolf & Walsberg 1996b). In this situation, evaporative water loss may account for 80% or more of the Verdins total water loss and can exceed 5% of a small birds body mass each hour (Wolf & Walsberg 1996b). Under these conditions the need to evaporate water in order to maintain body temperature below critical limits is in direct conflict with the maintenance of an adequate state of hydration (Webster 1991). The above attributes, combined with the high surface-area-to-volume-ratios of small birds means they are closely coupled to their physical environments and are must respond rapidly to changes in the thermal environment if they are to maintain homeostasis (Goudie & Piatt 1990). As a consequence, these animals may be under extreme selective pressures to optimize their use of water resources.
| Fig. 1. Maximun daily air temperatures for June of 1990 for Dateland, AZ (Azmet 1990). Temperaturas máximas del aire durante junio de 1990 en Dateland, AZ. |
Fig. 2. Total evaporative water loss rates for Verdins resting in the dark. Means are for 12 individuals at each data point, and 95% confidence intervals are shown at each temperature. From Wolf & Walsberg (1996b)
Tasas de perdida total de agua evaporativa para verdines descansando a la sombra. Promedios corresponden a 12 individuos para cada punto, y se presentan los intervalos de confianza del 95% para cada temperatura. En Wolf & Walsberg (1996b)
