Background/Question/Methods Avian haemoparasites (Plasmodium, Haemoproteus, Leucocytozoon) are widespread in free-ranging bird populations. Although most of these avian parasite taxa do not infect humans, understanding host-parasite interactions in such systems is important both for wildlife conservation reasons and to elucidate principles of infectious disease dynamics in natural ecosystems. Early researchers claimed that these microparasites had little effect on their hosts, but more recent work, in part from island ecosystems, suggests otherwise. Here we attempt to understand the effects of such infections on continental bird populations by studying natural populations of Mountain white-crowned sparrows (Zonotrichia leucophrys oriantha) in the mountains of the western U.S..

Results/Conclusions Over the course of a multi-year study, we identified a diversity of dipterans (including several mosquito and blackfly species) that are probable vectors at least 6 taxa of apicomplexan blood parasites (2 spp. of Plasmodium, 2 spp. of Haemoproteus and 2 spp. of Leucocytozoon) in the study population. To assess the impact of these pathogens on their hosts we manipulate haemoparasite loads by administering antiprotozoal agents. Our results indicate that parasite impacts on the host depend crucially on parasite species identity and that infections can have a variety of effects on the hosts. These include changes in plumage and song characteristics, but also significant reductions in reproductive output.