Current reproductive effort shapes the response to infection in a passerine bird.

When infected with a pathogen, a host may respond with an acute phase response to increase its chances of survival. Previous research shows that one fundamental component of the acute phase response, adjustments of body temperature, may differ depending on ambient temperature, social settings and energy availability. However, we do not know much about how such a response is modulated during work-intensive reproductive events. Therefore, we subjected breeding female blue tits Cyanistes caeruleus that were raising experimentally enlarged broods or normal-sized broods to a mimicked bacterial infection. We quantified their subsequent body temperature response and haptoglobin concentrations together with the effect on parental work rate and breeding performance. Immune challenged females with enlarged broods initially showed hypothermia while immune challenged females with normal sized broods instead developed fever. Additionally, immune challenged females with normal broods had higher circulating haptoglobin levels compared to immune challenged females caring for enlarged broods. Thus, females that were 'doubly' challenged with both an enlarged brood and an immune challenge mounted a lesser immune response but still managed to sustain nestling growth comparable to nestlings from normal sized broods. Hence, our results show that experimental manipulation of brood size shapes the acute phase response and the trade-off between self-maintenance and current reproduction.