Time—energy budgets (TEB) of Gambel's Quail (Callipepla gambelii) were compiled during two summers in the Colorado Desert of California. Quail spent 6.77 h/d foraging, 6.2 h/d inactive during daylight hours, and 11.02 h/d inactive at night. Field metabolic rate (FMR) calculated from this activity budget was 81.8 kJ/d. Of this, 47.3 kJ/d was expended during foraging, 12.6 kJ/d in daytime inactivity, and 20.4 kJ/d in nighttime inactivity. Despite the extremely hot thermal environment (maximum ambient temperature °45°C), there was no energy cost above resting levels for thermoregulation. FMR was also measured simultaneously with doubly labeled water (DLW), and averaged 90.8 kJ/d. TEB and DLW values agreed to within 6% when differences in measurement period were taken into account. A laboratory validation study indicated that DLW and balance methods agreed to within 5%. The FMR of C. gambelii was only 40% of that predicted for a bird of its body mass. This low FMR is primarily the result of a low resting metabolic rate (RMR): 51% of the predicted basal rate in 1981, and 70% of predicted in 1982. The basis and significance of this low and variable RMR are unclear. Energy assimilation efficiency, measured in laboratory feeding experiments with a mixed seed and arthropod diet, was 60.3%. An individual quail in the field thus required 150.3 kJ/d in its diet, representing a dry matter intake of 8.1 g/d. A diet of seeds alone provides insufficient water for Gambel's Quail in summer, so they must either incorporate moist food items in their diet or drink free water. It was calculated that over the course of a year, a population of Gambel's Quail consumes seeds with a total energy content °15% as great as that in seeds consumed by a population of desert rodents or harvester ants in the same area. Gambel's Quail thus may be important factors in the competition for resources among desert granivores, particularly because they can eat one of their competitors (harvester ants).

Chukars (Alectoris chukar) and Sand Partridges (Ammoperdix heyi), two ground—dwelling phasianids, are permanent residents of the Negev desert and are sympatric over much of their ranges. Sand Partridges (body mass = 150—250 g), however, inhabit only arid and very arid areas, whereas Chukars (mb = 350—600 g) are widely distributed and inhabit deserts only at the margins of their ranges. We compared some of the desert adaptations of these phasianids by measuring the seasonal field metabolic rates (FMR) and water influxes (using doubly labelled water), diet selection, and food requirements of free—living Chukars and Sand Partridges at a site where both species occurred. Both species showed adaptation in the form of low energy metabolism, which ranged from 43 to 81% of that expected for birds of similar body mass. During summer, Sand Partridges had lower energy expenditures (5.47 kJ°g— ⁰ . ⁶ ¹°d— ¹) and water influxes 72.3 mL°kg— ⁰.⁷ ⁵°d— ¹) than did Chukars (6.42 kJ°g— ⁰ . ⁶ ¹°d— ¹ and 93.5 mL°kg— ⁰ . ⁷ ⁵°d— ¹, respectively), indicating more pronounced adjustments to arid conditions in the desert specialist. However, both species obtained more than half of their water influx in summer by drinking. Their summer diet was relatively dry, consisting mainly of seeds (80%) along with some green vegetation (18%) and, in Chukars, occasional arthropods. This situation changed abruptly after winter rains, which induced germination and reduced the availability of seeds. Chukars were unable to maintain energy balance in the face of low ambient temperatures and a diet (90% green vegetation) that contained much water but comparatively little energy, and they mobilized fat reserves to meet energy requirements. Most Sand Partridges left the study area after winter rains, apparently migrating to the lower elevation, warmer, and drier Arava (part of the Rift Valley). The winter rainy season appears to be the most stressful time of the year for both species. The adaptations to hot, dry conditions possessed by Sand Partridges may be accompanied by constraints on their abilities to cope with cool, wet conditions, and this may restrict them to arid and very arid habitats.