From an eco-immunological perspective, the resistance to a pathogen is a costly trait for an organism, but efforts to qualify and quantify these costs are ambiguous. The humoral immune response is relatively slow and complex so that the quantification of its energy cost through a single time-point metabolic measurement is questionable. Here, we compared immune challenged and control zebra finches (Taeniopygia guttata) and estimated costs of the mounted humoral immune response through a combination of three different measurements, core body temperature, energy metabolism at rest, and markers for oxidative stress in the plasma. Body temperature was recorded continuously through implanted data loggers, whereas we measured resting metabolic rate and oxidative status only on day 7 post-challenge, at the expected peak of antibody production. The immune challenged birds revealed higher oxidative damage, a result that was not driven by the energy metabolism immediately measured before the blood sampling. Core body temperature, however, was higher during the first three nights in the challenged birds versus controls suggesting a higher metabolism during the initial phase of the immune response and was neither different after this, nor during the daytime throughout the experiment. The continuous measurement of body temperature provides indirect evidence for elevated energy metabolism associated with an immune response only in the very first days. In addition, elevated oxidative stress represents another cost associated with the immune response that is presumably driven by the energetic costs of this response.