The population crash of Icelandic Gyrfalcons from the 2021 HPAI outbreak continues with disproportionate mortality of young falcons as demonstrated by declines in annual territorial occupancy surveys and similar trends in national Christmas Bird Counts and eBird data. The majority of Gyrfalcon carcass recoveries have been discovered along the coast, where large numbers of seabirds are dying of HPAI, while surviving Gyrfalcons are mainly located inland where they are feeding predominantly on ptarmigan (80% of diet). We propose two research questions that will help improve our understanding of the Icelandic Gyrfalcon decline and guide science-based interventions to reverse the collapse of the population. First, are Gyrfalcon nestlings able to mount a protective immune response to a double dose of a commercially available HPAI vaccine? To answer this question, we will conduct a pilot HPAI vaccine trial on a small cohort (n=5) of wild Icelandic Gyrfalcon nestlings in Year 1 to evaluate immune response and quantify protection using serum neutralization. If the vaccine trial of nestlings is safe and confers protection, we will follow with field vaccination of nestlings in the northeast Gyrfalcon study area and adjacent territories in Year 2. And second, are surviving Gyrfalcons mounting a protective immune response to HPAI, and are diet and habitat associations risk factors for infection? We will examine this question by testing adult and juvenile Gyrfalcons for antibodies against HPAI to document seroconversion to vaccination and/or natural infection. As a corollary to our second question, we will compare immunocompetence with diet of Gyrfalcons to explore correlates to a seabird-rich diet. Dietary food preferences will be analyzed using remote camera trap technology to document food preferences and nestling survival. Additionally, we will fit GSM/GPS telemetry transmitters on a subset of juvenile Gyrfalcons (vaccinated and unvaccinated) to study post-dispersal mortality and movements.