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March 2017 Issue
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Blue duck

A whio resting on a South Westland mountain stream rock. Its bright yellow eye is plainly visible. Photo: Matt Winter
A rare treat on fast flowing streams in the North and South islands.

An iconic endemic bird, the blue duck/whio is a mountain stream dweller that requires a specific river habitat: a narrow but stable channel, coarse riverbed stones and gravel, high water clarity, shallow margins, pool and riffle sequences and forested margins.

Blue ducks are present in both North and South islands but are patchily distributed in rivers in forested headwater catchments along the main ranges. The pre-human range extended from high altitude tarns, lakes and rivers to sections of bush-edged lowland rivers and lakes.

Adult birds are uniformly slate blue-grey with chestnut spotting on the breast. Males have slightly more breast spotting and more prominent green iridescence on the head, neck and back. The bill is pale grey with a conspicuously expanded black flap across the tip which aids in gleaning food from round riverbed stones. The legs and feet are grey and the eyes are a beautiful vivid yellow.

A mature blue duck will be 50-55cm with males weighing around 900g and females 770g.

The well known, high pitched wheezy whistle call of ‘whi-o’ is made by the male bird. The female’s main call is a low rasping growl given when disturbed or as a threat.

The breeding season of North Island whio starts in August and runs through to January whereas the South Island population starts three weeks later. Nests will generally be close to the river edge, well hidden from above; sites include riverside caves, holes in river banks, at the base of fern clumps or beneath fallen trees. Five or six pale white eggs are usually laid, incubated by the female (while the male waits nearby on the riverside) over approximately 34 days, with chicks fledging at 75 days. The average productivity is 1.3 fledglings a year.

Any freshwater invertebrate is eaten, though chironomid (midge) larvae and cased caddis fly larvae are the most prominent. Berries from streamside plants have been recorded in the diet of alpine-dwelling ducks.