Avian influenza (AI) was first recognized as an avian pathogen in 1878, and since then, AI in poultry and wild birds has been extensively investigated. Despite the many studies that have been done to clarify the susceptibility and pathology of wild birds to H5N1 highly pathogenic avian influenza (HPAI) viruses, research addressing the susceptibility and transmissibility of low pathogenic avian influenza (LPAI) viruses in their natural host is still scarce. Information regarding inter-species differences in virus prevalence, as well as the distribution, ecology and life history of susceptible host species, is important in understanding AI virus epidemiology within the wild-bird reservoir system. The overall goal of this study was to provide a better understanding of host factors that affect the susceptibility of wild birds to AI viruses. Specifically, experimental trials were designed to evaluate if age-at-infection or host species affect the outcome of a LPAI virus infection in wild birds. In addition, the effect of a pre-exposure to a LPAI virus on subsequent LPAI or HPAI virus infections was evaluated. The results of these trials indicate that the susceptibility of wild birds to AI virus infection is complex and dependent on multiple factors, including AI virus isolate, age-at-infection, bird species, and pre-existing immunity to LPAI virus. The susceptibility and viral shedding pattern data of this investigation can be used as a baseline for future experimental trials with wild birds, and help to interpret data acquired in surveillance and field studies. Furthermore, this research contributes to the comprehension of the natural history of wild type avian influenza, and helps us define and understand the epidemiology of AI viruses in wild avian population.