Anas wyvilliana

Geographic Range

Hawaiian ducks ( Anas wyvilliana ), traditionally called koloas, are endemic to five of the Hawaiian Islands in the Pacific Ocean biogeographic region. They are found on the islands of Kaua'i, Big Island, Maui, southern Ni'ihau, and northern O'ahu. The current populations on Big Island, Maui, and O'ahu are present as a result of federal reintroduction efforts, though individuals in the Maui and O'ahu populations have likely hybridized with closely related species.

• Biogeographic Regions
• pacific ocean pacific ocean
  • native native

• Other Geographic Terms
• island endemic island endemic

Habitat

Hawaiian ducks depend heavily on wetlands for habitat, living in ponds, marshes, and inundated coastal grasslands. Hawaiian ducks also live in forested bogs or montane streams at elevations up to 3,300 m above sea level (ASL), although they more commonly live at elevations between 150 and 1,220 m ASL. The montane streams they inhabit are typically in areas with plentiful boulders, clear water, and dense native vegetation along the streambanks. Hawaiian ducks can also live in human-made ditches or streams with intermittent water flow. Efforts to conserve Hawaiian ducks include the construction of artificial wetlands with suitable levels of surrounding vegetation.

Hawaiian ducks forage in shallow waters, at depths less than 24 cm below the surface. Although they can live at higher elevations, Hawaiian ducks are only known to establish nest sites between 0 and 1658 m ASL. They build shallow, bowl-shaped nests lined with grasses and surrounded by dense vegetation, which helps camouflage nests from predators. Although they occasionally build nests over 1 km away from permanent bodies of water, they mostly nest along waterways, such as rivers or pond edges.

• Habitat Regions
• tropical tropical
• terrestrial terrestrial

• Terrestrial Biomes
• forest forest
• mountains mountains

• Aquatic Biomes
• lakes and ponds
• rivers and streams
• brackish water brackish water

• Wetlands
• marsh marsh
• bog bog

• Other Habitat Features
• riparian riparian

Physical Description

Hawaiian ducks exhibit sexual dimorphism in size and color. Females weigh 446.7 to 475.1 g and have wingspans that range from 210 to 232 mm, whereas males weigh 516.5 to 692.7 g and have wingspans measuring 213 to 238 mm. As adults, both sexes have chevron-shaped patterns of light and dark-brown coloration along most of their bodies. They have darker coloration on their breasts, flanks, and dorsal feathers. Adult males have dark, speckled chins and olive bills. Females are slightly lighter in overall color and have buff-colored chins with plainer dorsal feathers. Females also have buff-colored eyebrows with contrasting dark lines around their eyes.

Hawaiian ducks lay white eggs that are 48.7 to 59.6 mm long. Hatchlings have gray ventral feathers and dark brown dorsal feathers. Hawaiian ducks were once considered a subspecies of mallards ( Anas platyrhynchos ), but there are several morphological features that distinguish the two species. For example, mallards have larger wingspans, with lengths of 256 to 300 mm, and weigh almost twice as much as Hawaiian ducks. Furthermore, male Hawaiian ducks lack the bright green head plumage that male mallards have.

• Other Physical Features
• endothermic endothermic
• bilateral symmetry bilateral symmetry

• Sexual Dimorphism
• male larger
• sexes colored or patterned differently
• male more colorful

Reproduction

Hawaiian ducks are monogamous, forming long-term pair bonds. Females select mates based on flight displays, wherein three courting males fly vertically to a height of around 30 m and begin flying in small circles. If one male from the group is chosen, the remaining males continue the courtship display. Females typically prefer males that are strong fliers, which implies that males are selected based on their ability to defend territories. Hawaiian ducks establish pair bonds early in the breeding season, around November, and maintain their pair bonds until the end of the breeding season in May.

• Mating System
• monogamous monogamous

Hawaiian ducks typically breed once yearly. Breeding activity occurs year-round with a peak in the wet season, between November and May. Mating pairs generally breed once yearly. Females lay one egg a day until they have laid a full clutch, which can range in size from 2 to 10 eggs in captivity and 8 to 10 eggs in the wild. On average, clutch sizes are larger in the wet season (6.1 eggs) than during the dry season (4.5 eggs). Eggs have an incubation period of 28 days, (range 26-30 days).

Nest success is generally less than 40%, with most nest mortality occurring due to predation or flooding. Typically, only 3 to 4 eggs in a clutch hatch. Newly-hatched ducklings weigh 26.5 g on average and fledge at around 9 weeks old. Both males and females reach sexual maturity when they are around one year old.

• Key Reproductive Features
• iteroparous iteroparous
• seasonal breeding seasonal breeding
• year-round breeding year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual sexual
• fertilization fertilization
• oviparous oviparous

Female Hawaiian ducks are fully responsible for incubating their eggs, and they rarely leave the nest until eggs hatch. However, females do not feed their young after they hatch. Newborn Hawaiian ducks are highly precocial. They leave the nest on their first day and can fly by the time they are 9 weeks old. Ducklings are occasionally seen with their mothers before this time. Males help defend nest sites, but provide no direct parental care for their young after they hatch.

• Parental Investment
• altricial altricial
• female parental care female parental care
• pre-fertilization
  • protecting
    • male
• pre-hatching/birth
  • protecting
    • male
• pre-weaning/fledging
  • protecting
    • female
• pre-independence
  • protecting
    • female

Lifespan/Longevity

There is limited information regarding the longevity of Hawaiian ducks. However, closely related species such as American black ducks ( Anas rubripes ) are reported to live up to 26.6 years in the wild. It is likely that Hawaiian ducks have similar lifespans in the wild. Hawaiian ducks are not kept in captivity, so there is no information about captive lifespans.

Behavior

Hawaiian ducks are diurnal, dabbling ducks. They spend most of their time in bodies of water, but can also fly or walk on land. Although Hawaiian ducks do not migrate, they have been reported to fly between islands in the Hawaiian archipelago.

Hawaiian ducks are not known to have a consistent social structure, although they have been observed in groups of 5 to 15 individuals. They form monogamous pair-bonds, which usually last until females lay their eggs. Hawaiian ducks build nests, which average 27 cm in diameter and are found on the ground among dense vegetation. There is some evidence that males defend nesting areas, but females are the only ones that incubate eggs. Although Hawaiian ducks can breed year-round, peak breeding activity occurs between November and May.

• Key Behaviors
• terricolous
• flies
• natatorial natatorial
• diurnal diurnal
• motile motile
• sedentary sedentary

Home Range

Home ranges have not yet been quantified for Hawaiian ducks, but population densities on the island of Kaua’i are estimated at 2.5 birds for each km of stream length. Populations can reach higher densities around lentic waterbodies (e.g., ponds, lakes, marshes). Females defend their nest sites, which are an average of 27 cm in diameter. This equates to a nest territory of 572.6 cm^2.

Communication and Perception

Hawaiian ducks perceive their environment primarily using visual and acoustic cues. They are highly vigilant while active and use vocal calls to communicate with nearby conspecifics. Hawaiian ducks produce various vocalizations depending on the situation and the sex of the individual. Both sexes make soft quacking sounds, similar to that of mallards ( Anas platyrhynchos ). However, Hawaiian duck calls are higher pitched (mostly between 1.5 and 3.0 kHz) and less frequent, at intervals of about 0.7 seconds. Females typically quack louder than males, and males will also make hissing sounds in certain situations. Hawaiian ducks are usually quiet during the day and are more vocal in the early morning or late evening.

Hawaiian ducks also use visual cues to communicate, especially with potential mates. Groups of three males will fly in small circles around 30 m high to court females. Once a female selects a male, that male leaves the flight display, but the other males continue. Hawaiian ducks also use tactile communication when breeding or when foraging in shallow aquatic habitats.

• Communication Channels
• visual visual
• tactile tactile
• acoustic acoustic

• Perception Channels
• visual visual
• tactile tactile
• acoustic acoustic
• chemical chemical

Food Habits

Hawaiian ducks are omnivorous, feeding opportunistically on various food items. They consume algae, aquatic insects (both adults and larvae), crayfish, mollusks, seeds, and occasionally the tadpoles of marine toads ( Bufo marinus ). The seeds they eat usually come from grasses, such as rice ( Oryza sativa ) or native grass species.

Hawaiian ducks are considered to be dabbling ducks, meaning they do not dive to forage for food. They typically feed in waters less than 24 cm deep, and males and females share the same diet.

• Primary Diet
• carnivore carnivore
  • insectivore insectivore
  • eats non-insect arthropods
• herbivore herbivore
  • granivore granivore
  • algivore
• omnivore omnivore

• Animal Foods
• amphibians
• insects
• mollusks
• aquatic crustaceans

• Plant Foods
• seeds, grains, and nuts
• algae

Predation

Hawaiian ducks are most vulnerable to predation when they are in their nests. Wading birds such as black-crowned night herons ( Nycticorax nycticorax ) and cattle egrets ( Bulbulcus ibis ) consume eggs and nestlings. Many common nest predators are invasive species, such as feral cats ( Felis catus ), feral dogs ( Canis lupus familiaris ), small Indian mongooses ( Herpestes auropunctatus ), and black rats ( Rattus rattus ). Less common invasive predators include American bullfrogs ( Rana catesbeiana ) and largemouth bass ( Micropterus salmoides ).

As adults, Hawaiian ducks are relatively safe from predators, especially since hunting by humans ( Homo sapiens ) was prohibited. Females have cryptic coloration, possibly to help them avoid predation while they incubate nests.

• Anti-predator Adaptations
• cryptic cryptic

Ecosystem Roles

Hawaiian ducks are omnivores and thus may play a role in controlling the populations of the plants and aquatic animals they eat. They also serve as prey for birds, mammals, and some fish and amphibians.

Hawaiian ducks are documented hosts for parasites including the cestode species Cloacotania megalops and apicomplexan parasites in the genus Plasmodium . They are also hosts for various species of feather lice, including Trinotin querquedulae and Anaticola crassicorne .

Economic Importance for Humans: Positive

Hawaiian ducks were hunted for sport until it was made illegal, around 1939. However, illegal hunting may be ongoing. Currently, Hawaiian ducks are a source of ecotourism through birdwatching. They are one of only 6 species of waterfowl endemic to the Hawaiian islands, so Hawaiian ducks are popular among bird enthusiasts.

• Positive Impacts
• ecotourism ecotourism

Economic Importance for Humans: Negative

Hawaiian ducks have no reported negative economic impacts on humans. They have been reported to eat the seeds of rice plants ( Oryza sativa ), but not in large enough quantities to affect crop yields.

Conservation Status

Hawaiian ducks are listed as “Endangered” on the IUCN Red List and the U.S. Federal List. They are also protected under the U.S. Migratory Bird Act, but they have no special status on the State of Michigan List or in the CITES appendices.

Hawaiian ducks were originally listed as an endangered species in the early 1900s due to unsustainable hunting practices and genetic hybridization with mallards ( Anas platyrhynchos ). Hybridization with mallards can be detrimental, since it introduces traits to the gene pool that may not be beneficial in island environments. It is suspected that many of the extant Hawaiian duck populations have genetic material from mallards. However, Hawaiian ducks living at higher elevations are less likely to interact with mallards, and may be more genetically distinct. Other current threats to Hawaiian ducks include habitat loss and water pollution. Anthropogenic developments have reduced the amount of habitat available to Hawaiian ducks, and chemicals designed to kill aquatic weeds can have indirect effects on the health of adults and hatchlings. Furthermore, a number of invasive species - primarily mammals - prey on Hawaiian duck eggs and reduce nesting success.

There are several efforts in place to conserve Hawaiian ducks. Hunting was prohibited some time between 1939 and 1942. Although illegal hunting practices are still a threat, the reduction in hunting-related mortality has benefited Hawaiian duck populations. Hawaiian ducks were also part of captive breeding programs in the 1970s, and individuals from these programs were successfully reintroduced on several islands as late as 1989. Currently, populations on the island of Kaua‘i appear to be stable and self-sufficient. There is also a statewide recovery plan with the goal of lethally removing mallards and hybrids to limit further genetic exchange. Recent studies have estimated that around 2,200 non-hybrid individuals exist throughout the Hawaiian islands, but inter-island conservation efforts are needed to maintain the Hawaiian duck genome. Other conservation actions include invasive predator control at known nest sites.

Additional Links

Encyclopedia of Life

Contributors

Sierra Spencer (author), Radford University, Sierra Felty (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor), Special Projects.

References

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