Temperature Regulation in Nestling Cactus Wrens: The Development of Homeothermy

Abstract

The Cactus Wren is a permanent resident of semi-arid desert-scrub areas of the southwestern part of the United States and northem Mexico. In the Sonoran Desert near Tucson, Arizona, its breeding season usually extends from March to August, during which two and occasionally three broods of three or four young are raised. The Cactus Wren is an ideal subject for developmental studies because of its long nestling period ( 26 days ), convenient size, and accessibility. The development of structural and behavioral features of the Cactus Wren are described in detail by Anderson and Anderson ( 1961). and they were returned to their nests within 24 hours. Individuals were not used more than once. The young were fed ground beef and meal worms (Tenebrio larvae) and were maintained in a replete condition during the experiments. The ages of nestlings were usually known from our observations at nests and were confirmed by comparing wing length with a standard wing length growth curve ( Anderson and Anderson 1961) . Temperature regulation at ambient temperatures below 35°C was studied by placing nestlings in 260-ml plastic jars with air-tight lids. Air was passed through these containers at a.constant rate (one liter/min) after it was dried by passage through tubes of Drierite dessicant ( anhydrous calcium sulfate). Temperatures inside the containers were measured with indwelling mercury thermometers. The nestlings were isolate’d from the walls of the containers by nylon mesh so that a maximum amount of the body surface was exposed to the circulating air. A range of ambient temperatures was obtained by taking measurements at various times during the day throughout the breeding season. After the nestlings had been in the chambers for 45 minutes their body temperatures were measured by inserting a thermistor probe through the mouth deep into the proventriculus. Regulation at temperatures above 35°C was studied by placing nestlings in shallow paper cups in a constant temperature incubator ( ? 0.5C ). Body temperatures were measured over a three-hour interval at ambient temperatures of 36, 40, and 44°C. The relative humidity was not controlled but never exceeded 20 per cent. Temperatures of nestlings in the field were measured with orally inserted thermistors. Temperatures in the center of the nest cavities and in the shade outside the nests were measured with permanently fixed mercury thermometers.