Variation in immunity and health in response to introduced avian malaria in an endemic Hawaiian songbird

Emerging infectious diseases are spreading at unprecedented rates and affecting wildlife worldwide, with particularly strong effects on islands. Since the introduction of avian malaria to Hawaii a century ago, the disease has contributed to the decline and extinction of several endemic Hawaiian honeycreeper species. At low elevation, where avian malaria is prevalent, Hawaii Amakihi Chlorodrepanis virens honeycreeper populations have experienced strong selection by the disease and have evolved increased malaria resilience, the ability to recover from infection. The goals of this study were to describe malaria infection rates across Hawaii Island, to examine the role of innate immunity in malaria resilience, and to determine the effects of resilience and chronic infection on Amakihi health. We measured malaria infection and metrics of innate immunity and health in low elevation Amakihi populations, which are more resilient to malaria, and high elevation populations, which have experienced weak malaria selection and are less resilient to malaria. Avian malaria infection was higher at low than high elevation. Measures of innate immunity were higher in low than high elevation Amakihi, but also depended on whether the birds were from the leeward side of the island (low rainfall) or windward side of the island (high rainfall) and whether they were free‐living or captive. Regarding measures of health, body condition was better in malaria‐infected than uninfected Amakihi, while hematocrit did not vary by malaria infection but was higher in high than low elevation birds. These are among the first results to describe variation in immunity and health according to avian malaria selection and infection in Hawaiian honeycreepers. Understanding the impacts of introduced diseases on island species, which are particularly susceptible to novel pathogens, will provide critical insight into how introduced disease may affect endangered species. The goals of this study were to examine how innate immunity and health of the endemic Hawaii Amakihi are affected by avian malaria selection and infection, and to describe malaria infection rates across Hawaii Island. Malaria infection rates were higher at low than high elevation, and our results suggest that innate immunity is higher in areas where avian malaria selection has been strong (low elevation habitats). Regarding measures of health, body condition was surprisingly better in malaria‐infected than uninfected Amakihi, while hemat