Hawaiian Duck’s Future Threatened by Feral Mallards

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet 2007-3047

2007

nowhere else on Earth, and many 

of these species are declining or in 

danger of extinction. Although the 

Hawaiian Islands were once home to 

a remarkable diversity of waterfowl, 

only three species remain—the 

Hawaiian Goose (Nënë), Laysan 

Duck, and Hawaiian Duck (Koloa 

maoli)—all Federally endangered. 

The Koloa maoli is the only Hawaiian 

bird threatened by “genetic 

extinction” from hybridization with 

an invasive species—feral Mallard 

ducks. U.S. Geological Survey 

(USGS) biologists in Hawai‘i are 

working to find the causes of bird 

endangerment and ways to prevent 

extinction of the Koloa maoli and 

other threatened birds.

N

The Federally endangered Hawaiian 

Duck (Anas wyvilliana) (“Koloa maoli” 

or simply “Koloa” in Hawaiian), is an 

endemic species (found nowhere else on 

Earth) of the Hawaiian Islands. The greatest current threat to the future of the Koloa 

is cross-breeding or “hybridization” with 

the introduced Mallard (A. platyrhynchos). 

The Mallard was brought to Hawai‘i for 

farming, sport hunting, and pond beautification, but it escaped into the wild and 

became “feral.”

Koloa were once common on the main 

Hawaiian Islands. Until recently, scientists believed that more than 2,000 “true” 

Koloa remained, primarily on the islands 

early 70% of Hawai’i‘s native

of Hawai‘i, Kaua‘i, and Ni‘ihau. However, 

recent evidence shows that even these populations contain Mallard × Koloa hybrids.

The Koloa is a small dabbling duck 

similar in appearance to the Mallard but 

genetically distinct and differing in size, 

plumage, and behavior. The Koloa is a 

deeper, richer brown, more secretive, and 

about 20 to 30% smaller than the Mallard. 

The Koloa can be found from sea level to 

about 10,000 feet elevation in a diversity 

of wetland habitats. Habitat-use studies 

reveal that, in addition to lowlands, the 

Koloa frequents mountain wetlands and 

skillfully maneuvers winding river corridors and forest canopy, where human 

activity is minimal. However, little else is 

known about the breeding ecology, home 

range, movements, or population dynamics of this endangered duck. 

A Hawaiian Duck drake or male from the Island of 

Kaua‘i (copyrighted photograph by J. Denny). The 

duck’s name in Hawaiian is “Koloa maoli,” meaning 

“native duck,” which distinguishes it from introduced 

ducks or migratory ducks such as Koloa mäpu 

(Northern Pintail, A. acuta) or Koloa mohä (Northern 

Shoveler, A. clypeata) that winter in the Hawaiian 

Islands. Inset shows 10-day-old Koloa maoli duckling 

from Kaua‘i (photograph courtesy of B. Zaun).

The greatest threat to the future of the Koloa maoli as a unique species is cross-breeding with the introduced 

Mallard duck (A. platyrhynchos). This photograph shows feral Mallards, including “barnyard ducks,” at Wailoa 

River State Park on Hawai‘i. (Photograph by K. Uyehara.)

HawaiÔi Cooperative Studies Unit

University of HawaiÔi at Hilo

bird species are found
WHY IS CROSS-BREEDING WITH FERAL MALLARDS THE GREATEST 

THREAT TO THE FUTURE OF THE KOLOA?

Current distribution of Koloa × Mallard hybrids on the main Hawaiian Islands (data from Engilis and others, 2002, and U.S. Fish and Wildlife Service).

H y b r i d i z a t i o n

When a feral Mallard and a Koloa mate and produce fertile offspring, cross-breeding or “hybridization” occurs. 

Studies by biologists from many cooperating organizations, including the U.S. Geological Survey, suggest that 

interbreeding among these hybrids, Mallards, and Koloa 

is leading to loss of the Koloa as a unique island species.

The Greatest Threats—Introduced Species and Hybridization

Introductions of nonnative species can 

bring species together that were previously geographically isolated and had 

separate gene pools. This is particularly 

a problem when “aggressive” introduced 

species, such as feral Mallards, come in 

contact with rare native species.

Studies in other parts of the world have 

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NIÔIHAU

KAUAÔI

OÔAHU

MOLOKAÔI

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HAWAIÔI

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Hilo

Honolulu

PACIFIC OCEAN

Photo by A. Engilis

Photo  J. Jeffrey

Photo by A. Engilis

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Areas where Koloas once lived

Predominance of Koloa

Predominance of hybrids

Confirmed hybrids

Status of hybridization uncertain

Confirmed Mallard female and 

 Koloa male pair

EXPLANATION

Koloa

Koloa-like hybrids

Mallard-like hybrids

shown that feral Mallards are dominant 

ducks, adapted to survive and reproduce 

in habitats with human introduced predators. They also are well adapted to using 

habitats degraded by human activity. 

These factors may give feral Mallards a 

competitive edge over native ducks, such 

as the Koloa.

When a Mallard and Koloa mate, they 

produce fertile, hybrid offspring. Because 

of this, interbreeding among hybrids, 

Mallards, and Koloa can over generations 

lead to loss of the Koloa as a unique island species.

Mallards in Hawai‘i

Mallard imports to Hawai‘i began in 

the late 1800s for stocking ornamental ponds and farming. In the 1950s 

and 1960s, hundreds of Mallards were 

Feral Mallard male

U.S. Fish and Wildlife Service photo

female/male

female/male

female/male
Worldwide, native ducks are vulnerable to invasion and 

hybridization with feral Mallards, particularly when already 

in jeopardy from habitat loss and introduced mammalian 

predators. The Mallard occurs naturally in the Northern 

Hemisphere, but because of its popularity, people have 

transported it to areas where it was not previously found. 

Mallard introductions have caused conflicts with native 

duck populations throughout the world. In New Zealand, 

descendants of game-farm Mallards, estimated to number 

4 to 5 million, are responsible for the near extinction of the 

New Zealand Grey Duck. In South Africa, Mallards introduced by farmers and waterfowl collectors now threaten 

both the African Black Duck and Yellow-billed Duck.

Mottled Duck

Photo  M. Epstein

imported from North American game 

farms to stock some hunting areas in 

the Hawaiian Islands. Unlike “resident” 

feral Mallards, which are thought to be 

the descendants of such domestic stock, 

migratory Mallards that occasionally stop 

over or winter in the Hawaiian Islands are 

rare, not in breeding condition, and do 

not pose a threat to Koloa.

In addition to the “true” Mallard, 

there are Mallard domestic breeds or 

“barnyard ducks.” They come in various 

shapes, sizes, and colors and can breed 

with and are the same species as the Mallard. Common Mallard breeds found in 

Hawai‘i include the Pekin, Khaki Campbell, Blue Cayuga, Rouen, and Indian 

Runner. Barnyard ducks are bred for 

characteristics such as egg production or 

appearance. Although the chances of a 

Koloa and barnyard duck directly hybridizing in the wild are very small, many 

scientists have raised concerns about the 

large numbers of abandoned domestic 

ducks found on public water bodies and 

the possibilities that these ducks harbor 

bird and human diseases.

In the late 1980s, the Mallard was 

placed on the State of Hawai‘i’s List 

of Restricted Animals for importation, 

which allows exceptions for research and 

exhibition. In 2002, the Hawai‘i Department of Agriculture placed an embargo 

on all birds shipped to the islands (with 

some exceptions, for example, fertile 

eggs) to protect the public from West Nile 

Virus. However, the Mallard continues to 

reproduce and be sold in the State.

Resolving the Feral Mallard Dilemma in 

Hawai‘i

Removing the feral Mallard threat in 

the Hawaiian Islands is recommended 

by the U.S. Fish and Wildlife Service’s 

(FWS) draft recovery plan for the Koloa. 

However, even biologists have trouble 

distinguishing between Koloa, feral Mallards, and hybrids in the field, because 

the size and plumage of hybrids can 

vary greatly. To help with this problem, 

scientists at the University of California 

at Davis and the U.S. Geological Survey 

(USGS) are developing techniques to 

more accurately differentiate duck types. 

Hope for the Future of the Koloa

Koloa conservation involves not 

only gathering scientific data but also 

Koloa

Australian Black / 

New Zealand Grey Ducks

Yellow-billed Duck

African Black Duck

Meller’s Duck

American Black Duck

THE GLOBAL FERAL MALLARD THREAT

Photo by B. Zaun

Photo by C.D. Roderick

Photo  H. Stiver

U.S. Fish and Wildlife Service photo

Photo  H. Stiver

Photo  H. Stiver

Equator
Printed on recycled paper

For more information contact:

Michelle Reynolds 

U.S. Geological Survey

Pacific Island Ecosystems Research Center

Kilauea Field Station

P.O. Box 44, Bldg. 344

Hawaii National Park, HI, 96718

Email: michelle_reynolds@usgs.gov

http://biology.usgs.gov/pierc/

This Fact Sheet and any updates to it are available online at 

http://pubs.usgs.gov/fs/2007/3047/

Recommended Reading

Engilis, A., Jr., and Pratt, T.K., 1993, Status 

and population trends of Hawaii’s native 

waterbirds, 1977-1987: Wilson Bulletin, v. 

105, p. 142-158.

Engilis, A., Jr., Uyehara, K.J., and Giffin, J.G., 

2002, Hawaiian Duck (Anas wyvilliana), 

in Poole, A., and Gill, F., eds., The birds 

of North America, No. 694: Philadelphia, 

The Birds of North America, Inc.

Florida Fish and Wildlife Conservation Commission, 2004, A conservation plan for the 

Florida Mottled Duck: [http://myfwc.com/

duck/mottled/].

Rhymer, J., and Simberloff, D., 1996, Extinction by hybridization and introgression: 

Annual Review of Ecology and Systematics, v. 27, p. 83-109.

U.S. Fish and Wildlife Service, 2005, Draft 

revised recovery plan for Hawaiian waterbirds: [http://www.fws.gov/pacificislands/].

Research

• Determine range, behavior, and abundance of Koloa, feral Mallards, and their hybrids in Hawaii.

• Estimate the extent of hybridization between these species.

• Develop practical techniques for differentiating Koloa, Mallards, and their hybrids.

Planning 

• Develop a Koloa Conservation Plan to address the feral Mallard problem and use research results to guide management actions.

Recommendations

• Inform the public about the feral Mallard threat to Hawai’i’s endemic ducks.

• Ensure new stocks of Mallards and closely related ducks that threaten Koloa are not imported.

• Humanely remove existing feral Mallards and hybrids from the wild and prevent additional releases of captive Mallards.

informing the public and policymakers about the feral Mallard threat and 

developing humane methods for threat 

removal. The Koloa is protected by the 

Federal Endangered Species Act, but 

Mallard × Koloa hybrids are not.

Feral duck control programs in other 

regions have made significant strides 

toward conserving native duck populations, providing conservation models 

for Hawai‘i to follow. In Florida, for 

example, feral Mallards threaten the 

native Mottled Duck, and the State Fish 

and Wildlife Conservation Commission 

prohibits possession of Mallards (with 

exemptions for licensed shooting preserves 

and dog training), requires proper caging for 

permitted ducks, and prohibits sale of Mallards to anyone not properly permitted. 

Fortunately, once feral Mallards are 

removed, Hawai‘i’s isolated, island environment may provide natural barriers to 

reinvasion. The FWS describes the Koloa 

as having “a high potential for recovery.” 

Although little is known about the Koloa, it 

may be resilient in facing many of the threats 

that have extinguished other native Hawaiian 

birds. Given suitable Mallard-free habitat, the 

Koloa is expected to recover to self-sustainable breeding populations.

Koloa habitats in lowlands at Hanalei National Wildlife 

Refuge on Kaua‘i (top) and mountain streams in Kohala 

on Hawai‘i (bottom). Because Koloa and Mallards are 

thought to have similar needs, improvements to these 

habitats to benefit Koloa would also benefit feral Mallards and their hybrids. (Photographs by K. Uyehara.)

WHAT STILL NEEDS TO BE DONE TO ENSURE THE FUTURE OF THE KOLOA?

WHAT CAN YOU DO TO HELP 

THE KOLOA?

• Do not feed or shelter feral Mallard 

ducks. 

• Do not import, buy, or sell Mallards 

or their eggs.

• Do not release Mallards into the wild. 

Every Mallard released is a potential 

threat to Koloa.

• Consider having Mallards removed 

from your property. Contact the 

Hawai‘i Division of Forestry and 

Wildlife for advice.

• Be a responsible owner of domestic 

ducks, such as Pekin or Rouen, by 

keeping ducks penned, pinioned, 

or clipped, so that they do not 

leave your property.

• Share this information with others.

Kimberly J. Uyehara, Andrew Engilis, Jr., 

and Michelle Reynolds

Edited by James W. Hendley II 

Graphic design by Judy Weathers

COOPERATING ORGANIZATIONS

Hawai‘i Cooperative Studies Unit, 

Pacific Aquaculture and Coastal Resources Center, 

University of Hawai‘i at Hilo

University of California, Davis