Assessing natural dispersal of New Zealand bellbirds using song type and song playbacks

New Zealand’s managed offshore islands provide sanctuary to endangered and rare fauna but also benefit common native species. These productive islands may facilitate the expansion of mobile species back to the mainland. In northern New Zealand, many mainland protected sites are located on coastal headlands within short distances of these offshore islands. Bellbirds (Anthornis melanura), locally extinct on the mainland of this region for >100 years, are capable of dispersing these distances and are occasionally sighted along the coast. Nonetheless, it was unknown whether they had established breeding populations. Natural dispersal events are difficult to assess in terms of their source, structure and likelihood of succeeding. Females are generally more difficult to detect but when present provide conservation practitioners with confidence that a population may establish. Here we test a non-invasive monitoring method for a self-reintroduced population of bellbirds at Tawharanui, a managed coastal headland situated equal distances (20 km) from two potential source populations, Little Barrier and Tiritiri Mâtangi islands (LBI and Tiri, respectively). Bellbird song playbacks effectively confirmed the presence of both male and female bellbirds. The male and female song types recorded at Tawharanui were not found on Tiri but matched those of LBI and we propose this as the source population. We tested our playback protocol at other coastal parks and advocate annual playback surveys for detecting new populations at potential mainland sites.