Fever is an adaptive physiological response that animals use to fight infections by microorganisms. Although used routinely by veterinary and medical doctors for assessment of health status, there are hardly any studies of fever in free-living animals. Body temperature in a sample of more than 500 adult barn swallows Hirundo rustica varied considerably, but was consistent among capture events. Body temperature increased during the day, and reached a minimum in the middle of the breeding season. A normal quantile plot revealed that 4.5% of adults constituted a separate population that had fever. There were only marginal effects of handling on body temperature. Body temperature increased by 2.6 standard deviations following injection with LPS, showing that body temperature indeed increased with an immune challenge. Body temperature was negatively related to abundance of feather mites, but was not related to abundance of other ectoparasites or size of the uropygial gland. Barn swallows with high body temperatures also had large body mass and showed weak stress responses as reflected by their tonic immobility. Barn swallows in large colonies had lower body temperatures than solitary or less colonial individuals. Body temperature was not related to arrival date, timing of breeding, annual fecundity or adult survival. However, individuals that were easier to catch had higher body temperatures. These findings suggest that body temperature is a consistent physiological parameter of individuals, a small fraction of individuals has fever, and that febrile individuals have specific parasite loads, body mass, social environment and ability to escape capture.