Many factors influence the success of these new populations, including habitat quality, predators, capture and release techniques, the number and sex of individuals, and their genetic diversity. Now new research, the first of its kind, published in the British Ecological Society's Journal of Applied Ecology suggests bird song could also be important.
Ecologists from the University of Waikato and Lincoln University in New Zealand studied the North Island kōkako, an iconic bird with a haunting, organ-like song. Once widespread in the North Island, loss of habitat by deforestation and predation by rats, possums and stoats decimated the population.
By 1999, fewer than 400 pairs remained, and between 2001 and 2007, several pairs were moved from Te Urewera National Park to two other reserves: Boundary Stream Mainland Island and Ngapukeriki.
To find out how moving the kōkako has affected their song, the researchers made hundreds of recordings in the three populations and analysed differences in song using sonograms. They then used playback experiments to discover how birds from one population reacted to another populations' song.
They found the songs of translocated birds had diverged substantially from the source population, becoming less diverse with shorter and higher-pitched elements. According to Dr Laura Molles from Lincoln University: "Not only how kōkako sing in translocated populations, but also what they sing differs from kōkako in the source population."
The greatest changes were found in the population that had been translocated for longest, indicating the songs may become more different over time. But despite the divergence between each population's song, the playback experiments showed that the birds could not yet tell them apart.
"The songs diverge because birds such as kōkako learn their songs from parents, siblings and neighbours. As translocation usually involves only a small number of indivuals, they will take with them only a small portion of all the song elements in the larger source population. Subsequent variation in small populations will depend on that subset of songs and will then differ from the larger song pool in the source population," Dr Molles explains.
The study has important implications for conservation. Although in this study the kōkako populations have not been separated for long enough to cause song incompatibility, it will occur in time, the authors say. Once that happens, releasing additional birds into these populations could be problematic because song incompatibility could make interbreeding difficult.
As a result, says Dr Molles, conservationists should consider song variation as part of bird reintroductions: "We need to be aware that behavioural factors like song can also affect translocation success and recovery of endangered birds, and adapt our management of these populations accordingly. This means that we may have to work harder but the good news is that if we consider one more factor that we now know may also affect translocation, we will be more likely to succeed in conserving birds."
The North Island kōkako is one of New Zealand's most iconic bird species. The size of a common pigeon, both males and females have blue-grey plumage with black masks and striking bright blue wattles. Both sexes sing, and pairs duet, with a haunting voice and the birds' astonishingly varied organ-like notes can be heard over 1km away.
They have limited flying power, instead moving like squirrels through the branches and gliding from hill tops to valleys. They live in the temperate rainforest, feeding mainly on fruit and leaves. Once widespread, their numbers collapsed due to deforestation and predation by rats, stoats and possums, and by 1999 fewer than 400 pairs remained. Thanks to translocation to safe offshore islands, numbers have increased to around 800 pairs today.