An advantage of huddling in the cold is that the individual animals involved can maintain body temperature while saving energy. Since house sparrows Passer domesticus biblicus store little fat, but inhabit relatively cold climates, we tested the hypothesis that they huddle at night. While recording body temperature and body mass of 18 house sparrows when they were either caged individually, or free in an aviary, we observed that when free in the aviary, the sparrows huddled at low ambient temperatures and more birds huddled, in tighter and tighter formation, as ambient temperature decreased. However, their body temperatures were not significantly different from when they spent the night individually caged. When free to huddle, the birds lost significantly less body mass during the course of a day than when individually caged. This reduction in body mass loss may be of particular importance during periods of adverse environmental conditions, especially for small birds that manage their energy budgets on a daily basis.