The Use of Thermal Refugia by Two Small Desert Birds

Blair O. Wolf, Kenneth M. Wooden, Glenn E. Walsberg
1996 The Condor  
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
more » ... n 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. I METHODS AND MATERIALS Observations were made in a wash in the Goldfield Mountains. Pinal County, Arizona. at 473 m elevation. The wash is a normally dr; watercourse passing through Bulldog Canyon and its edges are populated by foothill paloverde (Cercidium floridurn), mesquite (Prosopis velutina), ironwood (Olneya tesota), catclaw acacia (Acacia greggi) and wolfberry (Lycium sp.) The surrounding hillsides and slopes are dominated by white bur-sage (Franseria dumosa), creosote bush (Larrea tridentata) and cactus (Carnegia, Opuntia, Ferocactus and Mammalaria). During late July and early August of 1995, one Verdin and eight Black-tailed Gnatcatchers were observed persistently occupying three very distinctive microsites on several afternoons during our occasional visits to the area. The physical location of the sites and the persistence with which the birds reoccupied them even in our presence suggested that occupancy of these sites may confer important thermal advantages. Air and substrate temperature measurements were made at each microsite using a Miller and Weber quick reading cloaca1 thermometer. These substrate temperatures represent a weighted value between the temperature of the surface and that of the surrounding air when the entire mercury bulb is not in contact with the surface being measured. As a consequence, our measurements of trunk surface temperature were somewhat higher than the actual temperatures of this substrate. Operative temperatures for Verdins were continuously measured in full and partial sun using five unheated taxidermic mounts in each microhabitat (Bakken et al. 1985). Operative temperatures for Verdins in deep shade were assumed to equal air temperature as measured by two 26 ga. copper-constantan thermocouples placed 1 m above the ground and 0.1 m from the trunk of a ironwood tree. Mount and thermocouple signals were measured five times a minute and averaged every 15 minutes by a Campbell 2 1 X datalogger. The activities of the Verdin were also recorded on two consecutive afternoons using an eventtiming program and a Hewlet-Packard 95LX palmtop computer mounted on a pair of binoculars. RESULTS AND DISCUSSION All three microsites were near the base of two large (approximately 50-70 cm DBH) paloverde trees situated near the center of the wash. Microsite 1 was located on the basal trunk of a paloverde, 10 to 40 cm from the ground, where two large subsidiary trunks joined (Fig. 1) . The tree' s dense canopy was approximately 10 m in diameter and hung to within 1 m of the ground. The tree was surrounded by catclaw and
doi:10.2307/1369162 fatcat:rsbcrpwsbvf63hzsnzeoblua7a