Blue duck

Song

The male's call is a high-pitched whistle.^[21][16] The female's call is a rattling growl or low-pitched grating notes.^[21][16][22]

Blue duck recording

Recording of adult pair in a stream communicating and answering tape recorded calls.

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Diet

The blue duck feeds almost entirely on aquatic invertebrate larvae. A study of blue ducks on the Manganuioteao River in the central North Island found the most common prey items were Chironomidae (midge) and cased caddisfly larvae, although cased caddisfly were less preferred and were only consumed so much because of their abundance. Hydrobiosidae (free-living caddisfly) and Aphrophila neozelandica (crane fly) larvae were also frequently eaten. Other prey included mayfly, Aoteapysche (net-building caddis) and stonefly larvae. In addition to these insects, it also consumes silt, periphyton and berries from riverside plants.^[23][24]

Foraging

Blue duck forages by dabbling on aquatic invertebrates in cold, fast, well oxygenated and clean mountain streams. It dabbles more by scouring rocks than dabbling at the water's surface. Studies have shown that adult territorial blue ducks dabble more than juvenile ones. The stream invertebrates are washed off the rocks in a downstream movement known as invertebrate drift. This drift keeps the food resource renewed for the duck in evenings. Another way to forage that is less commonly used by the blue duck is to dive in stream.^[25]

During the diurnal cycle, territorial birds exhibit higher foraging in the early morning and late evening while non–territorial birds tend to forage more at midday. The decrease in foraging by dabbling during midday is because of foraging by diving at that time. This pattern suggests that blue ducks capitalise on invertebrate resources that vary in availability during mid-day and morning or evenings.^[25]

Feeding behaviour is influenced by the type of prey available, requiring both tactile and visual foraging techniques. While foraging, blue ducks primarily glean invertebrates from rock surfaces using visual cues for mobile prey such as mayfly larvae. They have forward facing eyes that indicate this visual foraging use which is common of diving ducks. Predator absence had evolved the feature in it to use visual cues.^[26] Blue ducks also utilise more visual cues than tactical ones to scrape small chironomid larvae from submerged surfaces. Hence a disruption in water clarity in streams can significantly affect the duck's foraging patterns.^[27]

A higher availability of food resources makes the duck pairing to be less territorial because the habitat stability will be then increased.^[28] Although the food resources are critical for blue duck distribution and population structure, they do not necessarily affect the territorial size.^[29]

Male blue ducks are much larger than females and thus have significantly higher daily energy expenditures (DEE), but when size is controlled by expressing DEE as a multiple of basal metabolic rate (BMR), it is found to be equal in both. Ducks living in river channels at higher altitudes had relatively higher DEE than ones with higher channel lengths.^[29]

Territoriality

Blue duck groups keep a long territorial non–overlapping interval as seen in Manganuioteao River with a interval of 1 km. This behaviour secures access to essential resources and optimum breeding sites thereby enhancing reproductive success.^[28][25] During the breeding season, territory defence becomes critical as breeding pairs protect their territories from intruders. Both male and female blue ducks cooperate to defend their territories, using a variety of behaviours to deter intruders. Defensive behaviours include head-low flight, upright, head-bobbing, siphon-feeding and vocalisations. It is important for the pair to defend their territory from intruders because if one loses then the mating pair parts. Also males vocalise much more than female, usually with a "whio" call in morning.^[25][30]

Territoriality in blue ducks is linked to habitat quality. Reduction in invertebrate resource density leads to territoriality being no longer economically viable as defense requires higher food presence.^[29] While pairs defend their territories, extensive areas between them may remain unprotected. Juvenile and unpaired blue ducks exploit these undefended spaces for foraging thus showing that territoriality does not completely limit resource availability for unpaired individuals.^[25]

In the post breeding season, to reduce economic costs of defending their territories the blue duck pair would constrict their territory size to the most productive areas. Also in this situation, the territory was defended by only one individual from the pair. Most aggressive confrontations (rank=4) involve territorial males defending their territories against foraging intruders, highlighting male-male competition's role in shaping blue duck social dynamics.^[25][30]

Mating

For all year round, both of the monogamous adult pair of blue ducks defend their territories and rear the broods at the same time. After few mating seasons, if a stream reaches the carrying capacity for the ducks then a large number of contiguous territories will be held by closely-related individuals. Thus the mating behaviour of blue ducks leads to enduring pair bonds and permanent territories.^[28][30]

The ducklings when grown up usually loses contact with adult ducks when they are able to fly. In most cases, the moulting and thus successive flightless ability of the adult ducks come after the duckling learns to fly while exceptions do occur. Also if an unpaired male blue duck tries to displace paired mates, changes in mating occur twice–one during the period of incubation and the other during brood raising. These are the defensive actions against the intruder blue ducks.^[30]

Fully grown male adults (or Drakes) in a stable environment have higher reproductive variance than in an unstable environment where nest failures (by predators or female infertility) can be high.^[28] Also on an average, it takes 35 days for an incubation period and a clutch of 6 eggs are born in each blue duck family.^[30]

Breeding

From August to October, Blue Ducks lay 4–9 white eggs. The incubation period is of 35 days and chicks fully fledge after 70 days.^[19]

While nesting, they usually lay six eggs and breed within one year. Female incubate the eggs while the male guards the territory from intruders. The ducklings are reliable enough to hatch while weathering strong water currents. Both of the duck adult pair raises the duckling until the latter is well fledged.^[31]