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. (BirdLife International, 2017)
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. (BirdLife International, 2017; Engilis, et al., 2020; Malachowski and Dugger, 2018; Malachowski, et al., 2018; Paxton, et al., 2021; Raikow, 1974; Uyehara, et al., 2008)
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. (Engilis, et al., 2020; Malachowski, et al., 2018; Marshall, 2005)
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. (BirdLife International, 2017; Malachowski and Dugger, 2018; Malachowski, et al., 2019; Marshall, 2005)
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. (Engilis, et al., 2020; Malachowski, et al., 2019; Malachowski, et al., 2018; Marshall, 2005)
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. (Engilis, et al., 2020; Hawaii Audubon Society, 2005; Malachowski, et al., 2018; Marshall, 2005)
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. (BirdLife International, 2017; Engilis, et al., 2020; Wasser and Sherman, 2010)
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. (Engilis, et al., 2020; Malachowski and Dugger, 2018; Malachowski, et al., 2019; Marshall, 2005)
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. (Malachowski and Dugger, 2018; Malachowski, et al., 2019; Marshall, 2005)
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. (Engilis, et al., 2020; Marshall, 2005)
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 (Rhinella marina). 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. (Engilis, et al., 2020)
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 (Lithobates catesbeianus) 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. (Engilis, et al., 2020; Marshall, 2005)
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. ("A Health Survey for Hawaiian Duck (Anas wyvilliana) at Hanalei National Wildlife Refuge, Kaua‘i", 2016; Engilis, et al., 2020; Marshall, 2005)
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. (Engilis, et al., 2020; "Hawai‘i’s State Wildlife Action Plan", 2015; Marshall, 2005; Wasser and Sherman, 2010)
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. (Engilis, et al., 2020; Marshall, 2005)
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. (Engilis, et al., 2020; Fowler, et al., 2009; Marshall, 2005)
Sierra Spencer (author), Radford University, Sierra Felty (editor), Radford University, Karen Powers (editor), Radford University, Galen Burrell (editor), Special Projects.
body of water between the southern ocean (above 60 degrees south latitude), Australia, Asia, and the western hemisphere. This is the world's largest ocean, covering about 28% of the world's surface.
uses sound to communicate
young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
a wetland area rich in accumulated plant material and with acidic soils surrounding a body of open water. Bogs have a flora dominated by sedges, heaths, and sphagnum.
areas with salty water, usually in coastal marshes and estuaries.
an animal that mainly eats meat
uses smells or other chemicals to communicate
having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.
humans benefit economically by promoting tourism that focuses on the appreciation of natural areas or animals. Ecotourism implies that there are existing programs that profit from the appreciation of natural areas or animals.
animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.
parental care is carried out by females
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
an animal that mainly eats seeds
An animal that eats mainly plants or parts of plants.
An animal that eats mainly insects or spiders.
animals that live only on an island or set of islands.
offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).
marshes are wetland areas often dominated by grasses and reeds.
Having one mate at a time.
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
specialized for swimming
the area in which the animal is naturally found, the region in which it is endemic.
an animal that mainly eats all kinds of things, including plants and animals
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).
breeding is confined to a particular season
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
uses touch to communicate
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
uses sight to communicate
breeding takes place throughout the year
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