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Sociable Weavers Philetairus socius build large communal nests which they occupy year-round. White et al. suggested that use of this large nest complex significantly reduced the energy expenditure of Sociable Weavers during winter, thus lowering their resource requirements at that time. In this study we have used the doubly labelled water technique to examine the hypothesis that Sociable Weavers have a reduced field metabolic rate during the winter. Further, we measured their water influx and compared our findings with data from other desert-adapted birds. The field metabolic rate of Sociable Weavers (mean mass = 25.5 g) in the Kalahari Desert in winter averaged 48.7 ± 10.1 kj per day, a much lower energy expenditure than birds of similar size living in more mesic environments. We suggest that this results from a combination of communal roosting and possibly a reduced basal metabolic rate. The water influx of Sociable Weavers averaged 4.6 ml water per day, one of the lowest values reported for a bird of this size. Our analysis indicates that Sociable Weavers cannot rely on seeds as a food source without another source of water because the seeds do not provide enough water to meet their needs.