Identifying factors that determine the variation in disease risk in natural populations is of fundamental importance for understanding the ecology and evolution of host-parasite interactions and predicting infectious disease risk. In multi-host, multi-parasite systems, host species can vary substantially in infection risk and heterogeneity in disease risk among hosts can be driven by individual- or species-level host characteristics. At the species-level, variation in infection risk can occur because of differences in host-life history, behavior and environment that underpin patterns of parasite exposure [1–4]. At the individual-level, hosts can vary in infection risk owing to differences in exposure to parasites and host susceptibility. In the case of vector-borne diseases, hosts exposure to parasites can increase via increase in frequency of encounter with dipteran vectors that can influence disease transmission . For instance, host exposure can be impacted by geographical factors that affect vector abundance (e.g., elevation ) or host ecological traits that affect exposure risk such as foraging/nest height [7, 8] or sociality [9, 10]. Alternatively, some species-specific traits (e.g., sexual dimorphism) could be influenced by infection risk. Parasite-mediated sexual selection is an important mechanism favoring the evolution of secondary sexual traits (e.g., plumage brightness [11, 12]). Finally, parasite exposure risk can also affect individual-level factors associated with fitness (e.g., fluctuating asymmetry [13, 14] and body condition [15, 16]).
Avian haemosporidian parasites (Apicomplexa, Haemosporida) of the genera Plasmodium, Haemoproteus (including Parahaemoproteus) are protozoan blood parasites that affect bird populations globally . Avian haemosporidians (commonly referred to as avian malaria parasites) are an exceptionally diverse group of parasites, with over 2500 parasite genetic lineages . These parasites are transmitted by arthropod vectors, with Plasmodium being transmitted by Culicid mosquitoes, and Haemoproteus by Ceratopogonid biting midges and Hippoboscid louse flies [17, 19]. Avian haemosporidians can impose strong selective pressures on bird hosts as they are known to reduce longevity , host fitness [21, 22], individual host condition  and have led to severe population declines [24–27]. Previous research has revealed that avian haemosporidian parasites vary widely in their host range, with Plasmodium lineages often being generalists infecting a broad range of host species and Haemoproteus lineages often being specialists infecting one or few closely related host species [28, 29]. Plasmodium and Haemoproteus parasites also exhibit eco-evolutionary differences, with Plasmodium more affected by abiotic factors such as geography and Haemoproteus, primarily affected by biotic factors such as host phylogeny and host ecology . Given their widespread distribution, diversity and pronounced eco-evolutionary differences between Plasmodium and Haemoproteus, variation in parasite prevalence for the two parasite genera could be affected by different ecological factors in multi-host communities.
The Tropical Sky Island bird community in the Western Ghats mountains – located parallel to the southern coast of India (Fig. 2), offers an excellent model system to elucidate the factors influencing variation in avian haemosporidian infection risk. The Western Ghats are a global biodiversity hotspot , and the high endemic bird diversity in the Western Ghats  provides opportunities for native parasites to exploit a wide variety of hosts, allowing us to test how host ecology impacts parasite infection risk. Additionally, the landscape is threatened by anthropogenic habitat fragmentation and land-use changes; and the potential negative impact of avian malaria in this biodiversity hotspot makes the identification of factors associated with increased disease risk an important step for conservation .
Sky islands are isolated mountain-top habitats surrounded by dramatically different lowland habitats. The replicated arrangement of geographically discrete, identical habitats provides an ideal natural laboratory to explore ecological dynamics underlying avian haemosporidian infection risk. The Western Ghats Sky Islands hosts unique natural matrix of wet, montane evergreen forests and grasslands, locally known as Sholas, above 1400 m (henceforth Shola Sky Islands), while low elevations harbor drier habitats. High habitat heterogeneity and climactic conditions due to its elevational gradient have led to disproportionately high host species diversity in the Shola Sky Islands, comprising of host species having different habitat specialization, life history strategies and elevational distribution. For example, montane specialists are restricted to high elevations and generalists are distributed widely from high to low elevations. While montane specialists have likely been historically protected from avian malaria because low temperatures at high elevations leads to low vector abundance  or poor parasite development , this scenario is changing as global warming progresses . Thus, Western Ghats Sky Islands offer a valuable system in which to investigate disease dynamics, especially in the light of possible climate change driven extinctions in the landscape (e.g., Robin et al. ).
Although several factors have been proposed to explain variation in parasite prevalence and infection risk among individuals and host species [7, 8, 36–39], it remains unclear whether the role of host ecological traits are generally predictable or whether they are idiosyncratic across hosts, parasites and environmental conditions and context dependent. Additionally, few studies have taken evolutionary history of the hosts into account and thus, the importance of host evolutionary history in predicting infection risk is less well understood. Evolutionary history of host species can confound the relationship between ecological traits and parasite infection risk as closely related species are more likely to share risk factors compared to non-related host species .
In this study, we first examine the species- and individual-level ecological factors that influence variation in avian haemosporidian prevalence and thus avian haemosporidian infection risk in the Western Ghats. Next, we examine if these effects differ across the two parasite genera – Plasmodium and Haemoproteus. Second, we test the effect of host phylogeny in explaining variation in avian haemosporidian infection risk not explained by host ecological factors. Previous studies suggest that Plasmodium is a generalist parasite and Haemoproteus is a relatively specialist parasite [28, 29], thus we expect that the effects of ecological factors will vary for Plasmodium and Haemoproteus in addition to their intrinsic differences in parasite biology and vector specificity. At the species-level, we expect that (Fig. 1); 1) Species that have a lower minimum elevation will have higher Plasmodium prevalence whereas species with a higher minimum elevation will have higher Haemoproteus prevalence because of different environmental requirements of haemosporidian parasites that may limit their distribution on an elevational gradient; 2) Species foraging at higher forest strata will have lower Plasmodium prevalence and higher Haemoproteus prevalence compared to species foraging at the ground level because of vertical stratification in their arthropod vectors; 3) Social living species will likely exhibit higher parasite prevalence of both parasites as social living species may have a higher probability of encountering vectors and increase transmission risk; 4) Host species which exhibit sexually dimorphic traits will have higher haemosporidian prevalence because parasite pressure is a strong driver of sexual selection on these traits. Furthermore, at the individual-level, 5) birds with higher average body size will have a higher probability of infection for both parasites as larger body size will likely provide more surface area for vector feeding and emit higher quantity of olfactory cues (e.g., CO2), thereby attracting more vectors; 6) birds with better body condition will likely be less infected by both parasites compared to birds with poorer body condition that may have reduced immunocompetence (Fig. 1).