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This familiar large songbird is similar in size to a crow or a New Zealand pigeon. The white-backed form tyrannica is the largest of the sub-species. The male has a white hind-neck, mantle, rump and shoulder patches. The upper two-thirds of the tail and under-tail coverts are also white. The rest of the plumage is black, with a blue iridescence. The female is similar, but the mantle is grey, and the black parts of the plumage are less iridescent.
Both sexes have a blue-grey bill with a dark tip, and red eyes. The male takes several years to attain full adult plumage; after the second moult it resembles an adult female. Some white appears on the mantle after the third moult, and the remainder after the fourth moult. The juvenile is mottled grey on the under-surface.
The black-backed magpie is similar to the white-backed forms, but with a black mantle. The female can be identified by the presence of some grey on the lower hind-neck. The two subspecies interbreed, resulting in offspring with a varying amount of black on the mantle, ranging from a few feathers to a narrow band.
Magpies have a complex social system in which they form non-territorial or territorial groups. Magpies defend their territory by singing, aggressive posturing and fighting. This aggressive territorial behaviour makes them a pest.
They can be a considerable nuisance during the breeding season, swooping on and occasionally attacking people, especially children
Magpies can affect native birds by excluding them from breeding territories. They may also prey on chicks and eggs to feed to their young. Magpie control is likely to have significant benefits to the native bird population under these circumstances.