Blue duck

Song

The male's call is a high-pitched whistle.^[13][14] The female's call is a rattling growl or low-pitched grating notes.^[13][14][18]

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 Manganuiateao 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.^[19] The blue duck on occasion take berries and the fruits of shrubs.^[1]

Foraging

The foraging behaviour of the blue duck is characterized by its reliance on benthic invertebrates in fast-flowing riverine environments. Blue ducks primarily engage in dabbling behaviour where they feed by scouring rocks rather than up-ending or dabbling at the water's surface. Observations indicate that territorial males and females dabble approximately 96% of the time, with juvenile ducks dabbling 86% of the time. 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 dabbling during midday corresponds with an increase in diving behaviour in deeper pools. This pattern suggests that blue ducks capitalize on invertebrate resources that vary in availability throughout the day.^[20] Diving behaviour was seen most frequent in March and July when water levels are higher and prey living on stones and boulders above the water-surface would have been inaccessible using alternative foraging methods.^[21] Blue ducks’ foraging activities peak during early morning and late afternoon, coinciding with the diurnal activity patterns of invertebrate drift. The reliance on these temporal patterns allows blue ducks to exploit a resource that is continuously recolonizing denuded areas in the river.^[22] Ducks can be seen feeding at other times but often the day and night is spent in hiding or resting.^[23] Feeding behavior 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, typical of diving ducks, some attribute the evolution of this feature to the special absence of predators.^[23] Blue ducks also utilize tactile cues to scrape small chironomid larvae from submerged surfaces. ^[24] Changes in water clarity can therefore significantly affect foraging efficiency, particularly for mobile prey. Territoriality in blue ducks is closely linked to the availability of food resources. Although blue ducks occupy large territories, the size is not primarily determined by food abundance; rather, it reflects the overlapping life cycles of benthic invertebrates, which exhibit minimal seasonal variation. ^[22] This indicates that while food resources are critical for blue duck distribution and population structure, they do not necessarily dictate territorial size.^[25]

Territoriality

Blue ducks display territorial behaviours, they are known for their year-round territoriality, with territories typically ranging from 0.5 to 1.5 kilometres along river habitats characterized by alternating pools and rapids. The size and boundaries of these territories depend on resource availability, environmental conditions, and competition with other blue ducks.^[22][20] During the breeding season, territory defence becomes critical as breeding pairs protect their territories from intruders. This behaviour secures access to essential resources and optimal breeding sites, enhancing reproductive success. Both male and female blue ducks cooperate to defend their territories, using a variety of behaviours to deter intruders. Defensive behaviours include head bobbing, vocalizations, and aggressive confrontations. These behaviours communicate their presence and resolve conflicts over territory.^{[20] [26]} The intensity of these interactions varies based on several factors, including the presence of a female during confrontations. When females are present, aggressive behaviours may escalate, highlighting the significance of mate guarding in territory defence. This interaction shows social and environmental factors shaping territorial behaviour.^[20] Territoriality in blue ducks is linked to habitat quality. Research indicates that pairs in higher-quality environments expend less energy on territorial defence, allowing for greater foraging opportunities and improved reproductive fitness.^[25] While pairs defend their territories, extensive areas between them may remain unprotected. Juvenile and unpaired blue ducks exploit these undefended spaces for foraging, indicating that territoriality does not completely limit resource availability for non-breeding individuals.^[20] After the breeding period, blue ducks may adjust their territorial size in response to resource availability, often reducing their territory to focus on the most productive areas. As the breeding season progresses, interactions between neighbouring pairs can intensify, particularly with increasing competition for shared resources. Most aggressive confrontations involve males defending territories against foraging intruders, highlighting male-male competition's role in shaping blue duck social dynamics.^[20][26]

Mating

Blue ducks exhibit a complex social structure characterized by strong pair bonds and monogamous behaviour, integral to their reproductive success and territory defence in riverine habitats. Typically, blue ducks maintain permanent year-round territories defended by mated pairs, emphasizing the significance of monogamy for the successful rearing of offspring. ^[22] Pair members rarely stray far from each other, with individuals often remaining in close proximity throughout the year.^[26] In blue ducks, synchronous behaviours further enhance the stability of pair bonds, as pairs coordinate their activities, particularly during periods of brood dispersal and moulting. Pairs often forage together, displaying high synchronization rates during various behavioural activities, which can range from 62% during moult to 81% in the pre-laying phase. ^[22] This synchronized behaviour is said to likely improve foraging efficiency and reduces vulnerability to predation, reinforcing the significance of the social bonds. During incubation, males shift their foraging habits to support female partners and their offspring, highlighting a collaborative approach to parental care. ^[26] Although blue ducks generally exhibit monogamous pair bonding, instances of extra-pair mating may occur, particularly when environmental pressures or territory dynamics shift. ^[22]

Breeding

Blue ducks nest between August and October, laying 4–9 creamy white eggs. The female incubates the eggs for 31 to 32 days and chicks can fly when about 70 days old.^[16]

Nesting and egg incubation of four to seven eggs is undertaken by the female while the male stands guard. Nests are shallow, twig, grass and down-lined scrapes in caves, under river-side vegetation or in log-jams, and are therefore very prone to spring floods. For this, and other reasons, their breeding success is extremely variable from one year to the next.^[27]