Yellowhead  (Mohoua ochrocephala)

This small bird earns its common name for the beautiful, glimmering, vivid-yellow colouration that adorns its head and breast, while the remainder of the plumage is brown with varying tinges of yellow and olive (3). The female and juvenile are similar, but slightly less brightly coloured than the male, with the crown and nape more brown in colour (3) (4). The male's commonest call is a trill or rapid shivering rattle, which has been likened to the pea whistle note of a canary (2).

Endemic to New Zealand. Formerly widespread in the South and Stewart Islands, the yellowhead is now extinct on Stewart Island and has greatly declined on the South Island, with only a small number of fragmented populations left. Remaining strongholds exist in the Fiordland and Mount Aspiring National Parks (4).

Once found in podocarp/hardwood forests, the yellowhead now occupies beech forests, preferring those with fertile soils on river terraces, where food is abundant (3) (4). Nesting occurs in small cavities in large, old trees (4).

The breeding season of the yellowhead is late compared to other birds in its range, but varies with climatic conditions (2). Individuals breed from early October until March on the valley floor in the Eglinton Valley, Fiordland National Park, but those at higher altitude in Eglinton and elsewhere start later (5). Clutches contain one to five eggs, usually three, which are then incubated exclusively by the female for about 20 days (3) (5). In low altitude areas, most pairs raise two broods a year, but elsewhere a single clutch appears to be more normal (5). Although the female has sole responsibility over the lengthy process of incubation, both parents spend a relatively long time caring for the chicks after hatching (3). The life expectancy of the yellowhead is five years, although two wild birds have been recorded at at least 16 years old (4).

The yellowhead is primarily insectivorous, but occasionally supplements its diet with fruit when in season (4).

When Europeans first arrived in New Zealand the yellowhead was still abundant, but forest clearances, predation by introduced mammals and competition with introduced vespulid wasps (Vespula spp.) and birds all had a devastating effect on the species' survival (3) (6). As a result, this bird has disappeared from 75 percent of its former range (4). As with much of New Zealand's endangered fauna, habitat destruction has been a major cause of decline, with many forests still being cleared or modified by selective logging. Predation also poses a serious threat, with yellowheads suffering periodic population crashes when stoat (Mustela erminea) numbers erupt (4). These eruptions follow seasons of heavy beach seeding that occur every four to six years, which allow species up the food chain to multiply, with populations of insects, then mice, and then stoats, proliferating (4) (6). Stoats prey upon eggs, chicks and incubating adult females that are unable to escape from the nest hole, resulting in huge losses and a dramatically biased sex ratio (4) (6). Black rats (Rattus rattus) brought over on ships are also excellent climbers, and prey upon eggs, chicks and incubating females (3) (4). It is also thought that stoat control initiatives in certain areas may have reduced predation on the increasing rat population (3) (6). Competition for insects and honeydew from introduced vespulid wasps has contributed to the bird's disappearance from beech honeydew forests in the northern South Island, and introduced finches are thought to also be competing for food and contributing to the species' decline (6).

A monitoring programme established in 1983 carefully tracks 14 populations of yellowheads in 12 key sites, and stoats have been trapped in key habitats during years of heavy seeding (4). Survival rates in the Eglinton Valley, Fiordland National Park, have been closely followed for more than ten years to monitor whether stoat control efforts are having a positive impact, and initial results show that intensive trapping of predators does benefit the yellowhead (3). In addition, birds have been translocated to three mammalian predator-free habitats, including Codfish Island in 2003, where individuals proceeded to successfully breed the following season (4). This species has also bred in captivity at Orana Park in Christchurch, providing potential for future re-introductions into suitable habitats. Should re-introductions ever be deemed necessary, they would help to bolster wild populations of this beautiful, brightly-coloured endemic bird and greatly increase its chances of survival (3) (4) (6).

For more information on the Yellowhead see:

• BirdLife International:
  www.birdlife.org
• O'Donnell, C. (1993) (Mohua) Yellowhead Recovery Plan (Mohoua ochrocephala). Threatened Species Unit, Department of Conservation, Wellington, New Zealand. Available at:
  www.doc.govt.nz/upload/documents/science-and-technical/TSRP06.pdf

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