Much of global biodiversity is comprised of parasitic organisisms. It is well recognized that the selective pressures imposed by parasites shape host defenses and life-history strategies. Many studies suggest that human changes to the environment facilitate pathogen emergence by disrupting established relationships among parasites, their hosts, and their environment, as is often the case in human-modified landscapes. However, the lack of baseline knowledge of long-term, established host-parasite interactions in wildlife systems challenges our ability to make sound predictions on how future changes to the global environment will affect the health of human, domestic animal, and wildlife populations, as well as global biodiversity. Here we present data from multiple studies on the ecology of avian hemosporidian parasites in a common passerine population, the Mountain White-crowned Sparrow (Zonotrichia leucophyrs oriantha) breeding in a temperate alpine system. The first study (see Chapter two) is a broad sampling study that describes the avian host, potential biting dipteran vector, and blood parasite communities on our field sites. The second study (Chapter three) is a molecular study describing the Leucocytozoon community amplified from ornithophilic black fly species. This study (Chapter four) explores the avian host, dipteran vector, and environmental factors important for transmission of Leucocytozoon fringillinarum , the most prevalent parasite in the White-crowned Sparrow population. Finally, the fourth study (Chapter five) examines the effects hemosporidian parasites have on the White-crowned Sparrow population. We determine Leucocytozoon spp. were the most prevalent hemosporidian in the avian community, and that the black fly species Simulium silvestre/S. craigi had the potential to be a vector for a wide diversity of Leucocytozoon spp. in this system. We also established that relapsing infectious adults and young of the year birds are essential for seasonal persistence of Leucocytozoon spp. at these sites. Finally, infection with hemosporidians negatively affected bird populations by increasing stress levels and decreasing energy available to allocate towards other energetically expensive activities, such as reproduction.