The adaptive significance of many life-history traits of birds presumably can be understood within the context of patterns of energy allocation (Cody 1966, King 1974). Yet precious few data exist for the energy expenditure of free-living birds during their reproductive period. In particular, little is known about the energy requirements of females while they are feeding their young. Most extant information on the energetic cost of feeding young has been gathered indirectly by measuring the weight loss of parents as they feed different numbers of young (Hussell 1972) or from studies of caged parents (Brisbin 1969). With the advent of the doubly labeled water (DLW) technique (Lifson et al. 1955, Nagy 1975), reasonable estimates of the daily energy expenditure (DEE) of free-ranging birds now can be obtained (Nagy 1980). In essence, the technique involves isotopic labeling of an animal's body water with oxygen-18 and tritium or deuterium. From the difference between the turnover rates of the two isotopes, the rate of CO2 production can be measured. When we concurrently compared CO2 production in Savannah Sparrows (Passerculus sandwichensis) using DLW and standard laboratory techniques, we found a mean difference of +6.5% (range -0.2 to +11.0, n = 7), suggesting that acceptable estimates of CO2 can be obtained while these birds are functioning in their normal environment (Williams and Nagy 1984a). Furthermore, Nagy and Costa (1980) have shown that water flux rates estimated with tritiated water vary within ? 10% of actual flux rates in most situations. Using the DLW method, Utter and LeFebvre (1973) calculated that female Purple Martins (Progne subis) with nestlings metabolized an average of 183.6 kJ/ day (n = 2; mean wt = 47.7 g), which is somewhat higher than that of males during the same time period (142.9 kJ/day; n = 2). Males apparently did not feed nestlings as much as did females. Unfortunately, the number and age of the nestlings were not reported. Hails and Bryant (1979) found that female Common House-Martins (Delichon urbica, 20 g) feeding young metabolized 75.3 kJ/day, but the DEE of female birds was not correlated with brood mass. In this study, we used DLW to measure the DEE of female Savannah Sparrows while they were feeding nestlings. Specifically, we wanted to compare the DEE of females while feeding a normal brood of 3 young with those feeding 2 young, late in the nestling period when energy demands presumably are greatest. The study area and birds.-Our study area lay in the middle and upper littoral zone of a large salt marsh located on the Point Mugu Naval Air Station, Pt. Mugu, California (34?07'N, 119?07'W). The vegetation consisted of, in decreasing order of importance, Salicornia virginica, Frankenia grandifolia, Batis maritima, and Monanthochloe littoralis. Large, barren salt pans and shallow tidal channels intermixed with patches of vegetation to form a mosaic of sites used by Savannah Sparrows for foraging and nesting. The breeding biology of the Savannah Sparrow at Pt. Mugu has been studied extensively by J. B. Williams and will be reported in detail elsewhere. In brief, these small, ground-nesting passerines reside there year-round and commence breeding in early April. Males defend territories (ca. 0.1 ha) and aid in provisioning the young, but only females incubate eggs. Of 83 nests found in 1978 and 1979, 78% contained 3 eggs, 18% held 4 eggs, and only about 4% held 2 eggs. Mean clutch size for these 83 nests was 3.16 ? 0.46 SD. In this study we removed a nestling from broods of 3 to make broods of 2 young several days prior to DLW measurement. Field and laboratory methods.-We first determined the impact that capturing and handling females had on their subsequent behavior and the time required after disturbance for females to resume normal parental activities. To this end, we gave the birds sham injections of distilled H2O and restrained them in a cloth bag for 1 h. We then released the female bird and watched from a blind for her return. In most cases, females handled 3-4 h before sunset resumed caring for young within 2-3 h after their release; however, about 20% of the females that we netted abandoned their young. All birds for which we present data in this study resumed feeding their young within 3 h after their release. Each female was colorbanded to facilitate recognition. We captured birds by placing mist nets around their