Whio – the blue duck

The whio/blue duck is one of New Zealand’s rarest and most endangered endemic ducks in mainland New Zealand. It is an iconic species and appears on the $10 note.

The whio’s scientific name is Hymenolaimus malacorhynchos, which refers to its unique soft bill. It has no close relatives in the world but does belong to the Anatidae family (also known as waterfowl), which also includes other ducks, geese and swans.

The whio is a type of torrent duck, which are specially adapted to living year round in fast-flowing rivers and streams. These streams are far too swift for most ducks. In fact, the whio is New Zealand’s only true torrent duck and is one of only four torrent duck species in the world.

They are found mostly in the back country where there are clean, fast-flowing, stony-bottomed rivers and streams shaded by riparian bush cover. This limits them to places such as Te Urewera, Central North Island and the West Coast of the South Island from Fiordland to Kahurangi National Park, although they were once more widespread. There is a genetic difference between the South Island and North Island populations, but they can’t be distinguished by sight.

Biology of the whio

The whio is a blue-grey duck with a speckled chestnut breast. It has a white/pale grey bill with soft black flaps at the tip. Its legs and feet are dark grey. It has distinctive yellow eyes that face forward.

Both sexes are very similar, although the males are larger. The main way to distinguish the sexes is by their voices. Male whio have a high-pitched whistle – ‘whi-o’ – and the females call with a low rattly growl. Young whio are similar to the adults but with dark eyes and less chestnut on their chests.

Whio generally live year round as territorial pairs. They stay with the same partner throughout the year, and many mate for life. They hold and defend territories along suitable rivers, and once established, many territories are held for life. The average territory extends about 1 km along the sides of a suitable waterway.

The whio feeds almost entirely on the aquatic invertebrates found on rocks in clean rivers. It has a bill uniquely designed for feeding in rocky waterways. The bill has a pair of soft black flaps on each side that assist in scraping the invertebrates off the rocks and protect the bony bill from damage. Most foraging is in shallow rapids, but whio also dive for their food and sometimes eat drifting larvae and the adult invertebrates from the water surface. The ducklings feed on the same foods as the adults and can dive in shallow water within days of hatching.

Whio nest close to the river’s edge in log jams, caves, holes in the riverbank, clefts in rocks and in dense riparian vegetation. They make shallow nests filled with accumulated debris and breast feathers.

North Island whio/blue ducks begin nesting and then laying eggs in August through to November, with the first ducklings usually emerging in late September. In the South Island, whio begin laying about 3 weeks later. The pale creamy-white eggs are laid 1–2 days apart, and the average clutch is five or six eggs. The female whio incubates the eggs for about 35 days while its partner stands guard.

Baby whio hatch with green bills, which then develop the final pale colour after about 8 hours. The ducklings are born able to negotiate white water and forage for food straight away. The ducklings are covered in down feathers, which are waterproofed by natural oils.

Both parents look after the ducklings during the first 70–80 days. The parents will brood ducklings by tucking them under their wings and body to keep them warm, particularly at night, and will protect them until they fledge. Juvenile ducklings fledge when they have developed most of their contour and flight feathers and are able to survive on their own. The young ducks gradually disperse from the territory when their parents start their post-breeding moult. All ducks replace their feathers annually. Whio are unusual in that they moult over a particularly short period of time, leaving them especially vulnerable.

Young whio will attempt to establish territories close to where they were hatched, and this results in strong genetic patterning between rivers.

Ecological niche

Whio occupy a unique ecological niche – they are extremely well adapted to fast-flowing, cold, stony-bottomed river and stream environments. These environments support freshwater ecosystems that include a variety of other species such as eels, fish, aquatic plants and macroinvertebrates.

Whio feed on a diversity of macroinvertebrate larvae from species such as mayflies, stoneflies, caddisflies and dobsonflies. These species feed on biofilm and algae off the rocks and have physical adaptations confining them to colder freshwater environments, pristine rivers and streams with consistent water flow and little or no sediment. They are considered indicator species for healthy, clean waterways as they cannot survive in waterways containing large amounts of sediment or pollution. Sediment clogs the gills of these larvae, making it hard for them to obtain oxygen from the water.

Where these species live, whio are also able to survive, and so whio are also considered an indicator species. If whio are present, the stream or river is considered to be clean and healthy.