Experimentally activated immune defence in female pied flycatchers resultsin reduced breeding success

Traditional explanations for the negative fitness consequences of parasitis m have focused on the direct pathogenic effects of infectious agents. Howev er, because of the high selection pressure by the parasites, immune defence s are likely to be costly and trade off with other fitness-related traits, such as reproductive effort. In a field experiment, we immunized breeding f emale 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 repro duction and self-maintenance. Immunized females decreased their feeding eff ort and investment in self-maintenance (rectrix regrowth) and had lower rep roductive output (fledgling quality and number) than control females inject ed 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 breedi ng success. This may be caused by an energetic or nutritional trade-off bet ween immune function and physical workload when feeding young or be an adap tive response to 'infection' to avoid physiological disorders such as oxida tive stress and immunopathology.