The Use of Thermal Ref’ugia by Two Small Desert Birds’

The extent to which birds select specific microsites during their diurnal activity and the thermal consequences of microsite selection during the day has received scant attention (DeWoskin 1980; Walsberg 1985, 1993: Weathers and Sullivan 1989). Most research has focused on microsite selection and the physiological consequences of occupancy of specific sites such as nocturnal roosts or nest sites (Walsberg 1985). Diurnal microsite selection based on thermal constraints may have important effects on other daily activities such as foraging, social behavior, reproduction and the avoidance of predators. Observations of the curtailment of these activities due to thermally imposed physiological constraints is thus important to understanding the behavioral ecology of birds in desert environments. The general problems presented by terrestrial environments are carried to an extreme in subtropical deserts. In such areas, surface water is rare, humidities are low, and intense solar radiation produces very high air temperatures during the four to five month-long summer. For example, air temperature-which is best regarded as a minimum index of environmental temperature-sometimes exceeds 50°C and commonly is above 35°C for over 12 hours each day in the Sonoran Desert. Most birds are diurnal and do not use burrows for refuge, thus they must directly confront the hottest periods of the summer day. Small passerine birds also have high mass-specific metabolic rates, resulting in high rates of internal heat production and frequent ventilation of respiratory surfaces with resultant high rates of pulmonary water loss (Dawson 1982). In addition, their small size confers a low thermal inertia and limited capacity for storage of vital resources such as water. Such species must balance water budgets over time periods of minutes to hours (Goudie and Piatt 1990) and may be under extreme pressures to optimize their use of water resources. In this paper we report observations of diurnal microsite selection during the summer in the Sonoran Desert for two small (five to seven g) insectivorous birds. the Verdin (Auriparus flaviceps) and the Blacktailed Gnatcatcher(Pol~optila~melar&a). We also evaluate the potential effects of occupancy of these sites on avian water economy and rates of evaporative water loss.