15 2 16 ABSTRACT17 Parasites are a selective force that shape host community structure and dynamics, but 18 host communities can also influence parasitism. Understanding the dual nature from 19 host-parasite interactions can be facilitated by quantifying the variation in parasite 20 prevalence (i.e. the proportion of infected host individuals in a population) among host 21 species and then comparing that variation to other ecological factors that are known to 22 also shape host communities. Avian haemosporidian parasites 23 (e.g. Plasmodium and Haemoproteus) are abundant and widespread representing 24 an excellent model for the study of host-parasite interactions. Several geographic and 25 environmental factors have been suggested to determine prevalence of avian 26 haemosporidians in bird communities. However, much remains to be known regarding 27 whether host and parasite traits, represented by phylogenetic distances among species 28 and degree of specialization in host-parasite relationships, can influence parasite 29 prevalence. The aims of this study were to analyze factors affecting prevalence in a bird 30 community and to test whether the degree of parasite specialization on their hosts is 31 determined by host traits. Our statistical analyses suggest that prevalence is mainly 32 determined by the interaction between host species and parasite lineages where 33 tolerance and/or susceptibility to parasites plays an essential role. Additionally, we 34 found that although some of the parasite lineages infected a low number of bird species, 35 the species they infected were distantly related and therefore the parasites themselves 36 should not be considered typical host specialists. Prevalence was higher for generalist 37 than for specialist parasites in some, but not all, host species. These results suggest that 38 prevalence mainly results from the interaction between host immune defences and 39 parasite exploitation strategies wherein the result of an association between particular 40 parasite lineages and particular host species is idiosyncratic. 3 41 Keywords: bird community, Haemoproteus, host-parasite interaction, Plasmodium, 42 prevalence. 4 43 INTRODUCTION 44 Parasites have been suggested as a selective force since they might shape host 45 community dynamics [1], alter interspecific competition and influence energy flow [2]. 46 An essential trait in studies of host-parasite interactions is prevalence (i.e. the 47 proportion of individuals infected by a parasite or pathogen in a population at one point 48 in time) [3]. The importance of prevalence is reflected in the amount of studies showing 49 how major ecological factors shaped its intra-and interspecific variation [4,5]. In bird 50 communities, geographic areas, environmental issues or host species/population have 51 been suggested as factors determining prevalence [6]. However, more studies are still 52 needed in order to understand mainly factors affecting parasite prevalence at the 53 community level. 54 Avian Plasmodium and Haemoproteus spp. rep...