Elevated reproductive effort increases blood parasitaemia and decreases immune function in birds: a meta-regression approach

1. In recent years there has been much interest in physiological trade-offs involving host immune function and parasite defence, with the suggestion that they could play a pivotal role in mediating well-documented life-history trade-offs, such as the cost of reproduction. 2. Among studies of birds, the hypothesized link between reproductive effort and parasite defence has received particular attention, yet support for a trade-off between these two traits remains equivocal. 3. We used meta-regression analysis and an information-theoretic approach to investigate, among avian studies, how strong the effect of experimentally altered reproductive effort is on (i) infection with blood parasites from four common genera (Haemoproteus, Leucocytozoon, Trypanosoma and Plasmodium) and (ii) the ability of hosts to mount an immune response to novel antigenic challenge. 4. Across studies, there was a relatively weak but well-supported positive effect of reproductive effort on blood parasite infection levels. Importantly, this effect was significantly influenced by the parasitological measure employed; where parasitaemia (proportion of parasitized cells within infected hosts) was used as the response variable, effect size was almost three times as large as where infection prevalence (presence vs. absence of infection among hosts) was measured. 5. A moderate negative effect of reproductive effort on immune responsiveness was also found across studies. This effect was greater the longer the time that had elapsed between manipulation of reproductive effort and measurement of immune responsiveness, and was also related to the stage at which reproductive effort was manipulated, with manipulation during brood rearing producing stronger effects than manipulation during incubation. 6. Overall, these results provide evidence that reproductive effort can have pronounced effects on both parasitism and immune responses, but that effect size is influenced by methodology - what is measured and when. Exactly how such effects arise and whether they are sufficient to provide a mechanistic explanation for the cost of reproduction remains to be fully explored.