Experimental removal of the male parent negatively affects growth and immunocompetence in nestling great tits.

Given the available empirical evidence on the benefits and costs associated with immune defence, a role for the immune system in the trade-off between current and future reproduction has been predicted. This hypothesis was studied in a free-living population of great tits (Parus major) by examining the effects of male removal on the immunocompetence, body condition, and recapture probability in the widowed females and their nestlings. Furthermore, we investigated whether growth and its relation to immunocompetence were affected in the nestlings. For a short-lived species such as the great tit, one could predict that widowed females will compensate for the lack of any male assistance in feeding of their chicks and that they consequently might jeopardize their own health. However, we did not find any negative effects of male removal on body mass or condition, nor on humoral immunocompetence against sheep red blood cells in the widowed females by the end of the feeding period. In contrast, we observed significantly reduced body mass and size as well as a reduced T-lymphocyte cell-mediated immune response (expressed as the thickness of the swelling to a subcutaneous injection with phytohemagglutinin) in the experimental nestlings compared to the control nestlings. In addition, the experimental nestlings showed a tendency for a reduced chance to be found breeding the following year. Furthermore, our results showed that in the experimental nestlings, which suffered from unfavourable growth conditions, tarsus length was inversely related to cell-mediated immunocompetence, whereas in control nestlings this relationship was significantly positive. The relationship between cell-mediated immunity and body condition was found to be significantly positive in the experimental nestlings while in control nestlings there was no relationship between both variables. The latter finding suggests different priorities of investment in body condition between different growth conditions.