Functional Ecology 2009, 23, 405–415 doi: 10.1111/j.1365-2435.2008.01507.x © 2008 The Authors. Journal compilation © 2008 British Ecological Society Blackwell Publishing Ltd Elevated reproductive effort increases blood parasitaemia and decreases immune function in birds: a meta-regression approach Sarah C. L. Knowles* 1 , Shinichi Nakagawa 2 and Ben C. Sheldon 1 1 Edward Grey Institute, Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK; and 2 Department of Zoology, University of Otago, PO Box 56, Dunedin, New Zealand Summary 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. Key-words: avian malaria, brood size manipulation, cost of reproduction, haematozoa, immuno- competence, meta-analysis, PHA Introduction The cost of reproduction (Williams 1966) is a central tenet of life-history theory and posits that individuals face a trade-off between current and future reproduction. Thus increased investment in the current reproductive attempt is expected to be offset by a reduction in either survival or future fecundity (Stearns 1992). Reproductive costs have been documented in diverse taxa (Reznick 1985). In birds, numerous field experi- ments have indicated that this trade-off can be important in the currency of either survival (Dijkstra et al. 1990; Golet et al. 1998; Visser & Lessells 2001) or future fecundity (Gustafsson & Sutherland 1988; Hanssen et al. 2005). However, the physiological mechanisms that underpin this trade-off remain under debate (Zera & Harshman 2001). Although several mechanisms have been proposed, including the energetic costs of moult (Siikamäki et al. 1994; Nilsson & Svensson 1996) and tissue damage arising from oxidative stress (Nilsson 2002; Alonso-Alvarez et al. 2004; *Correspondence author. E-mail: sarah.knowles@zoo.ox.ac.uk