In 1987, conservationists brought 2 male owls to Norfolk Island for its last surviving female; their descendants later rebuilt the population, but inbreeding still limits recovery

Every Norfolk Island owl alive today descends from a conservation decision made in 1987. Back then, a small group of people took a chance and flew two male owls from New Zealand, hoping to get at least a couple of baby owls. They had few options because only a handful of owls remained on the island. By the time scientists confirmed that only one owl remained, there were no other Norfolk Island boobooks left for it to mate with. The Norfolk Island boobook owl was close to becoming completely extinct in the eighties because most of the large hollow trees had been cut down, leaving only a couple of owls with nesting sites.

One female and a rescue plan no one had tried before

The surviving female had no other relatives on the island. Her closest living relatives were members of a related subspecies in New Zealand. In 1987, conservationists brought two male birds from New Zealand to the island and prepared nesting boxes for any pair that might need them. The decision was unusual because it meant that the last female of a pure kind would be mated with males from another, but similar, New Zealand subspecies rather than with a genetically identical male, which she did not have. It was the only option that gave the subspecies hope of surviving.


Rebuilding a population from almost nothing

The female paired with one of the two males, resulting in hybrid offspring being born in 1989 and 1990, and subsequently bred in later years, gradually recovering the population. Conservation efforts, continuing since the 1987 introduction, have taken decades to help the owls re-establish themselves within the remaining forest on the island. The population today is between 45 and 50 birds, with heavy inbreeding. The rescue prevented extinction, but the population never developed enough genetic diversity to be genetically healthy.

Why a shrunken gene pool is holding back full recovery

Every owl now on Norfolk Island is descended from that founding pair, one female and one male, since the second introduced male failed to breed. As a consequence, the new subspecies possessed too little genetic diversity to have a reasonable chance of persisting in the long term. Small, isolated populations of any species tend to exhibit inbreeding depression, wherein the surviving members of the population carry disadvantageous alleles which may reduce the population’s fertility and viability, lowering the population’s growth rate long after the threat has ceased; in fact, there has not been a single successful breeding in the Norfolk Island population since 2012. However, the more insidious problem with the Norfolk Island population is genetic constraints inherent in the rescued population.

Nest boxes, competition, and the daily work of recovery

Conservation efforts on the island have continued since the 1987 introduction. Because the genetic pool cannot be restored, conservationists focus on giving each breeding pair the best possible conditions to raise healthy offspring. Maintaining the nest box programme, which predates the 1987 introduction, is also important. Because forest reserves on Norfolk Island have been reduced, natural nesting habitat for the birds has been greatly decreased. Introduced crimson rosellas compete with the native owls for the limited number of nest boxes, and reducing this competition is a priority in the recovery plan for the species.

A recovery that is still not finished

The Norfolk Island boobook owl example shows the long-term aftermath of an emergency rescue program. A population of 45 to 50 birds is still small, and when the species reached its lowest point, it was on the verge of extinction. The population was not fully restored to its previous state and still faces extinction risk.