The yellowhead or mohua (Mohoua ochrocephala) is a small insectivorous passerine bird endemic to the South Island of New Zealand. Once a common forest bird, its numbers declined drastically after the introduction of rats and stoats, and its IUCN conservation status is now near threatened.

Taxonomy

The yellowhead was formally described in 1789 by the German naturalist Johann Friedrich Gmelin in his revised and expanded edition of Carl Linnaeus's Systema Naturae. He placed it with the flycatchers in the genus Muscicapa and coined the binomial name Muscicapa ochrocephala. The specific epithet is derived from the Ancient Greek ōkhros meaning "pale yellow" and -kephalos meaning "-headed". Gmelin based his account on the "yellow-headed fly-catcher" that had been described in 1783 by the English ornithologist John Latham in his book A General Synopsis of Birds. The naturalist Joseph Banks had provided Latham with a watercolour painting of the bird by Georg Forster who had accompanied James Cook on his second voyage to the Pacific Ocean. The inscription on the painting includes the year 1774 and the location "Queen Charlotte's Sound". Queen Charlotte Sound is on the north coast of the South Island of New Zealand. This picture is the holotype for the species and is in the collection of the Natural History Museum in London. The yellowhead is now one of three species placed in the genus Mohoua that was introduced in 1837 by the French naturalist René Lesson. The species is monotypic: no subspecies are recognised. It is also known as mohua from the Māori . Recent classification places this species and its closest relative, the whitehead, in the family Mohouidae.

Distribution and habitat

The yellowhead is found only on the South Island and several small islands surrounding it. Although abundant in the 19th century, particularly in southern beech forests on the South Island and Stewart Island / Rakiura, mohua declined dramatically in the early 20th century due to the introduction of black rats and mustelids; nesting in tree holes makes them more vulnerable to predators.

Today they have vanished from almost all of the South Island's forests and exist in less than 5% of their original range. The proportion of time used in foraging is highest during the premoult period and is lowest during the incubation period. In wild conditions, caterpillars (Lepidoptera larvae) were the predominant food. In captivity, mohua were observed to prefer mealworms (Tenebrio molitor larvae) over other foods. This correlated with greater foraging on the trunk and large branches of forest trees as opposed to the twigs, foliage and small branches during October. However, this behaviour is an evolutionary adaptation to the lack of resources, which is beneficial if there is an abundance of mohua. Like other birds, mohua weave cup-shaped nests using soft materials such as moss, grass, filmy ferns and lichens. The tree cavities used were mainly on the trunk, but dead branches with rotten wood were also utilised. If the female mohua does not get enough food, the breeding season will be short and late. These flocks can sometimes consist of many mohua families, which causes the flocks to be louder overall. More releases are planned to islands in Fiordland. The population of mohua in the Landsborough valley has increased four-fold since 1998, thanks to an intensive programme of pest control, including aerial 1080. The population is now strong enough for birds to be transferred out to establish a new population on Resolution Island.<!-- WP:RS needed; removed link to WP:SPS (Twitter, FB, WP)--> Similar aerial 1080 operations in the Catlins and the Hurunui, Hawdon and Eglinton valleys have had equally encouraging results. DOC and TBfree New Zealand noted in March 2014 that there was a significant repopulation in the Catlins of other avian species, including bellbird and tomtit due to the reduction of predators.

Since the redesign of New Zealand's currency in 1991 a yellowhead has appeared on the reverse side of the New Zealand $100 note.