Endothermic animals in the boreal region experience considerable thermoregulatory costs in winter, especially during inactive periods when high demands cannot be met by increased food intake. This might necessitate a trade-off between resting thermoregulation and other energetically expensive processes, such as immune function, but this has rarely been investigated. We evaluated how immune activation affected rest-phase hypothermia (an important energy conservation mechanism in small animals), in wild great tits (Parus major L.) wintering under semi-natural conditions in southern Sweden. The acute phase response was triggered immediately prior to sunset through intramuscular lipopolysaccharide (LPS, an endotoxin) administration in two contrasting, naturally occurring, thermal regimes (cold, mild). We then noninvasively measured the thermoregulatory response throughout the night using subcutaneously implanted temperature-sensitive transmitters. Despite the energetic benefits of hypothermia, immunized great tits displayed an attenuated hypothermic response manifested through the onset of fever, possibly at the expense of a higher sustained metabolic rate. In line with this, these birds lost more body mass overnight (but only in mild conditions) and consequently were in worse condition the subsequent morning. Fever expression was remarkably similar in cold and mild conditions, despite substantial variation in the energetic cost for thermoregulation. This suggest that the degree of fever is flexible, and represents a trade-off between the minimum body temperature required to mount an adequate acute phase response and the energy expenditure needed to maintain it. We suggest that while energy conservation and immune function may not always be exclusive activities, costs of immune defences probably constitute an important additive energetic cost during northern winter conditions, sometimes calling for trade-offs with other important physiological processes to survive the night.