Genomic studies have shown that introgressive hybridization is a common phenomenon across the tree of life, particularly among young radiations. As incipient speciation tends to be induced by vicariance events, it is assumed that introgressive hybridization is more frequent in young species radiations in which allopatrically distributed species have a high probability of comming into secondary contact. In this study we draw on whole genomic data to investigate spatio-temporal introgression patterns in a songbird radiation that has colonized a highly dynamic island region in the Indo-Pacific. Some taxa within this radiation have colonized remote oceanic islands whereas others occur on landmasses and islands in the Sahul region that were at times connected during Pleistocene periods of low sea level. Our results show that introgressive hybridization has been pervasive within this young radiation, despite prominent plumage differences between taxa. Geographical proximity has been an important factor for hybridization and we further find that species occupying islands in the environmentally unstable Sahul region exhibit particularly high signatures of introgressive hybridization. Yet, one species appears to have been shielded against hybridization, perhaps due to specific ecological specializations. Finally, we identify a hybrid species on an island where two oceanic radiations meet. Collectively, our results support a growing body of literature that suggests that reticulate speciation is much more common than previously thought. This has implications for our understanding of how species form and how species are maintained through time.

Haemosporidians are among the most common parasites in birds and often impact negatively host fitness and consequently can affect entire host populations. It is therefore important to determine what factors influence parasite prevalence, particularly if they are caused by anthropogenic activities. Here we explore the effect of temperature, forest cover, and proximity to anthropogenic disturbance on haemosporidian prevalence and host-parasite networks on a horizontal spatial scale, comparing four fragmented forest patches and five localities within a continuous forest in Papua New Guinea. We find that the majority of Haemosporidian infections are caused by the genus Haemoproteus and that avian-haemosporidian networks are more specialized in continuous forests. At the community level, only forest cover was negatively associated with Haemoproteus infections, while abiotic and anthropogenic effects on parasite prevalence differed between bird species. We further tested if prevalence and host-parasite networks differed between the canopy and the understorey (vertical stratification) and found significantly higher Haemoproteus prevalence levels in the canopy, and the opposite trend for Plasmodium prevalence. This implies that birds experience distinct parasite pressures depending on the stratum they inhabit, likely driven by differences in vector communities. These three-dimensional analyses of avian-haemosporidians at horizontal and vertical scales provides a deeper understanding of the environmental factors driving haemosporidian prevalence in tropical lowland forests of New Guinea. Collectively, our results suggest that the effect of abiotic variables on haemosporidian infections are species specific, and that factors influencing community-level infections are primarily driven by host community composition.