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Determining how an animal's social context alters its immune responses will help us understand how pathogens impact individual health and spread within groups. Several studies have shown that group-housed animals can suppress components of the acute phase immune response, specifically sickness behaviors like lethargy. However, we do not know whether individuals alter sickness behaviors or other components of the acute phase response, including thermoregulation, in response to the infection status of other group members. We used automated radio telemetry on captive house sparrows (Passer domesticus) to test whether sickness behaviors and thermoregulation differed during immune challenge under 2 social contexts: 1) all of the flock inoculated with lipopolysaccharide (LPS), a non-replicating component of gram-negative bacterial cell walls, or 2) half of the flock inoculated. We predicted that with half of the flock inoculated, LPS-treated birds would be under more pressure to maintain competitive behaviors, so would suppress components of the acute phase response. As we predicted, LPS-inoculated birds showed less pronounced heterothermia (fever) when housed with a mixture of inoculated and healthy flockmates. In contrast, LPS-inoculated birds exhibited similar degrees of lethargy regardless of the infection status of their flockmates. Our results show that the infection status of an individual's social group did exert an effect on the acute phase response but surprisingly did not impact the expression of lethargy, a canonical sickness behavior. Determining the mechanisms underlying these responses will require testing additional social contexts with different ratios of infected to uninfected birds. [ABSTRACT FROM AUTHOR]