Traditional explanations for the negative fitness consequences of parasitism have focused on the direct pathogenic effects of infectious agents. However, because of the high selection pressure by the parasites, immune defences are likely to be costly and trade off with other fitness–related traits, such as reproductive effort. In a field experiment, we immunized breeding female flycatchers with non–pathogenic antigens (diphtheria–tetanus vaccine), which excluded the direct negative effects of parasites, in order to test the consequences of activated immune defence on hosts' investment in reproduction and self–maintenance. Immunized females decreased their feeding effort and investment in self–maintenance (rectrix regrowth) and had lower reproductive output (fledgling quality and number) than control females injected with saline. Our results reveal the phenotypic cost of immune defence by showing that an activated immune system per se can lower the host's breeding success. This may be caused by an energetic or nutritional trade–off between immune function and physical workload when feeding young or be an adaptive response to ‘infection’ to avoid physiological disorders such as oxidative stress and immunopathology.