Hybridization increases measures of innate and cell-mediated immunity in an endangered bird species.

1. Given the increasing threat of 'emerging infectious diseases' (EIDs), and the vulnerability of small inbred populations to such novel challenges (at least partly through compromised immunity), a degree of interspecific hybridization may improve the viability of endangered species by enriching depauperate gene pools. 2. To investigate this hypothesis, we quantified indicators of the strength of both innate and cell-mediated immunity in wild populations of hybridizing parakeets on the Chatham Islands, New Zealand. 3. We show that both measures of immune function are markedly higher in the cosmopolitan red-crowned parakeet Cyanoramphus novaezelandiae than in the island endemic Forbes' parakeet C. forbesi, as is to be expected when comparing relatively outbred vs. relatively inbred species. 4. In addition, we show that both measures of immune function are higher in Forbes'x red-crowned parakeet hybrids than in the Forbes' parakeet. Even those individuals with few red-crowned characteristics, suggesting they are the products of matings between hybrids and Forbes' parakeets, have significantly higher measures of immune function. 5. As at least one of the immune indicators measured correlates directly with both survivorship and the viability of small bird populations, this study reveals a potential management option for hybridizing species of conservation concern, where an increase in resilience to novel disease challenges could outweigh some loss of genetic integrity.