Genetic rescue attempt in a small, inbred population of a wild endangered passerine

Translocations can facilitate gene flow between fragmented populations, introducing new genetic variants that may mask deleterious alleles and relieve inbreeding depression (i.e., ‘genetic rescue’). Genetic rescue attempts in threatened populations are underutilised due to uncertainties surrounding outbreeding depression, appropriateness of source and recipient populations, and the duration of beneficial effects. The Aotearoa New Zealand hihi (Notiomystis cincta) is a threatened passerine hampered by disease outbreaks and reproductive failure. We assessed the long-term outcomes of a genetic rescue attempt, ten years after 20 immigrant hihi were translocated from Te Hauturu-o-Toi Island, the last natural population, to a reintroduced population on Tiritiri Mātangi Island. There was marginally higher multilocus heterozygosity in the source individuals, and low genetic differentiation, based on FST, between the populations, suggesting low risk of outbreeding depression. Six immigrants successfully fledged offspring, but only three have descendants in the current population. Following translocation, we observed short- and long-term increases in microsatellite-based neutral genetic diversity and increases in functional diversity across three Toll-like receptor (TLR) loci associated with innate immunity. Over ten years, survival to hatch decreased with proportion of immigrant ancestry, but individuals with immigrant ancestry were more likely to fledge and recruit into the breeding population. In the short term, F1 immigrant offspring were less likely to hatch, but more likely to fledge and recruit indicating potential heterosis at later life stages. However, differences in the probability of early-life survival between individuals with local and immigrant ancestry declined in the F2 onwards. Inbreeding increased following translocation, likely because one immigrant contributed a disproportionate number of descendants. Benefits of translocating individuals may be limited by the high degree of stochasticity in immigrant establishment. In species like the hihi with high levels of male-male competition, establishment may be improved by translocating females or pre-reproductive young into recipient populations.