Feral Cats are a Serious Threat to Hawai‘i’s Endangered Birds, Monk Seals and Dolphins

Cats killing birds may seem a normal, if mildly distasteful, part of nature. But many cats also carry a microscopic parasite that causes toxoplasmosis, an infectious disease that threatens Hawai‘i’s endangered birds, monk seals and even dolphins. New Zealand and Australia have aggressive programs to eradicate invasive animals, including feral cats. How do we solve our cat problem?


Cat vs Birds Cover Spread

Illustration: Michael Byers



Diane Halford loves her two cats.


She loves the little black one with white paws that her grandson named Mittens. She loves the older one with green eyes named Coco who always seemed to slip between her legs and outside the front door whenever Halford arrived home, her arms full of groceries.


Halford, a retired nurse who lives with her husband and two cats in busy Mānoa, loves her cats so much she decided to keep them indoors. “My daughter would say, ‘But Mom, Coco wants to go outside.’ And I’d say, I know but I worry about her. I worry about someone trying to poison her, and I worry about her getting hit by a car.”


What’s more, Halford keeps chickens in her backyard along with a rabbit. And then there is her concern for Hawai‘i’s native species. “I love animals, and I love the environment. I want to help preserve the animals that are at risk. I know there are albatrosses at Ka‘ena Point, but I don’t know exactly what the situation is in Mānoa.”


It turns out there are native birds in Mānoa, and in many residential neighborhoods across Hawai‘i. Kōlea, or Pacific golden plover, which have a strong Hawaiian cultural connection, forage in grassy yards and parks. White-tailed tropicbirds nest in the cliffs lining Mānoa Valley. While native, neither are listed as species of concern for extinction by the International Union for the Conservation of Nature and Natural Resources.


But the O‘ahu ‘amakihi is.


Well-known as one of Hawai‘i’s honeycreepers, a family of birds that evolved from a finchlike ancestor into 50-some unique species—two-thirds of which are now extinct—the O‘ahu ‘amakihi is a rare native forest bird that nests in residential communities, Mānoa included.


Then, there are the manu-o-kū, white terns, designated the official bird of Honolulu and listed as threatened by the state of Hawai‘i. In the main Hawaiian Islands, this small seabird is found only on O‘ahu, and only in the urban and suburban areas along the southern shore.


I love animals, and I love the environment. I want to help preserve the animals that are at risk.”
— Diane Halford


After some online research, Halford and her husband turned a portion of their backyard into a catio, a screened patio for cats. It didn’t take much. For a few hundred bucks, Coco and Mittens now enjoy the great outdoors without any worry about them getting harmed or harming others. For cat lovers, catios may be the latest craze since Hello Kitty sprang onto the scene. In hipster Seattle, Washington, and Portland, Oregon, there are even catio home tours.


Diane's cat in a catio

Diane Halford’s cat Mittens plays inside of their Mānoa home catio. Photo: David Croxford




Sure, we know cats and birds don’t mix. It’s a problem that often forces animal lovers to take sides—cats or birds.


The problem isn’t limited to heavily populated O‘ahu. On Kaua‘i, high in the remote mountains surrounded by terrain so rough that biologists get dropped in by helicopter, cats have been caught on infrared cameras crawling into the ground-nesting burrows of Hawaiian petrels and Newell’s shearwater and dragging out chicks and adults alike. On Hawai‘i Island, on the slopes of Mauna Kea, cats have killed the critically endangered palila, a large finch with a golden-yellow head and breast.


Shoreline populations get hit, too. Regularly, reports are made about cats preying on coastal wedge-tailed shearwater nesting colonies across the state, as well as Hawai‘i’s endangered waterbirds—the Hawaiian coot, the Hawaiian moorhen, the Hawaiian stilt and Hawai‘i’s state bird, the nēnē, or Hawaiian goose.


In 2014, the U.S. Department of the Interior released its “State of the Birds” report, which dubbed Hawai‘i the “bird extinction capital of the world.” Seventy-one known bird species in Hawai‘i have gone extinct. Another 33 are on the brink. And one-third of all bird species listed as endangered in the United States are found across the Hawaiian archipelago.


More than 100 years ago, in 1903, English naturalist R.C.L. Perkins first implicated cats in the killing of native birds. “On Lāna‘i,” he wrote, “in walking up a single ravine, I counted the remains of no less than 22 native birds killed by cats.”


At least 20 native bird species have gone extinct since Perkins’ report.


Obviously, cats aren’t the only predators here. Yet even if every cat owner went out and built a catio today, there would still be an estimated 300,000 lost, stray and feral cats roaming O‘ahu alone, making it seem as if little has been done to curb the killing since Perkins stumbled upon two cats devouring their prey. Even with mandatory sterilization of outdoor cats. Even though 87 percent of Hawai‘i residents want to see the number of free-roaming cats reduced, according to a research study by the University of Hawai‘i. The discussion about what to do about all these cats is a delicate one. As sensitive as, say, asking a friend if they’re voting #withher or the other guy in next month’s election.



But what if it wasn’t just birds being taken out by cats? New studies coming out of the scientific community suggest Hawai‘i’s killer-cat problem is even larger than we thought.


In early 2015, a pregnant Hawaiian monk seal, tagged RB24, miscarried. She was later sighted in shallow waters off Ko Olina, reluctant to haul out on the beach or swim. Within a month, she was dead. She was 8 years old.


Laboratory testing confirmed mortality due to toxoplasmosis, causing inflammation of the brain and lungs, necrosis of fat tissues and the aborting of her unborn pup. RB24 was the eighth known endangered Hawaiian monk seal to die of toxoplasmosis. With a population of fewer than 1,300 throughout the entire Hawaiian archipelago, it was enough to cause concern.


“Toxo hasn’t really been detected until this past decade, but the cases seem to be increasing,” says Charles Littnan, Ph.D, lead scientist for the Hawaiian Monk Seal Research Program. “It’s a limited sample size, but toxo has already proven to be as lethal as fisheries interaction. We don’t have a solution for the care of animals with toxo. By the time they’ve started demonstrating signs we can detect in behavior, we may have already lost the race for that animal.”


Later in 2015, a male Hawaiian spinner dolphin stranded itself on the beach at Kauna‘oa along Hawai‘i Island’s Kohala Coast. In reviewing a photographic database, scientists realized they knew the animal due to his distinctive dorsal fin. It was L414, first sighted in 2011. Reports are that, when L414 stranded, beachgoers pushed him back into the water. The next day, snorkelers found his lifeless body in the shallows.


When Kristi West, Ph.D, opened up the dead animal’s body, she found, “Very enlarged and inflamed lymph nodes, indicating the body was fighting something.” West is an associate professor of biology and the cetacean stranding director at Hawai‘i Pacific University. “We suspected an infectious disease.”


But West wasn’t sure exactly what disease until test results came back from independent labs to which samples had been sent. “Every tissue tested positive confirming fatal disseminated toxoplasmosis. It wreaked havoc in this animal and ultimately resulted in his death.”


Toxoplasmosis is an infectious disease caused by a microscopic protozoan, a single-celled parasite, known as Toxoplasma gondii. The disease is spread through the feces of the family of felids, cats, the definitive host. When a cat ingests the parasite, usually through infected prey, the parasite reproduces in the cat’s digestive tract, eventually excreting millions of oocysts in the cat’s feces. These oocysts become infectious within a couple of days and can survive for a year or more. It just takes one oocyst to infect another animal, such as a bird, as it accidentally ingests the parasite while grazing or eating an infected insect. The bird then becomes an intermediate host. It won’t produce oocysts in its scat, but its infected body can spread the disease to its predator. If that predator is a cat, the whole process starts all over again.


If anyone besides scientists is really aware of toxoplasmosis, that’s pregnant women, because the Centers for Disease Control advises against them changing kitty litter, letting pet cats outdoors and handling stray cats, because toxo in humans can cause miscarriages and birth defects.


But how is a land-based parasite killing marine mammals? Littnan and West figure it’s the same way sea otters along the central coast of California were dying off in the 1990s. The parasite makes its way to the sea by way of inland waterways and runoff.


“Toxo is a true mauka-to-makai problem,” Littnan says. “Even cats way up in the mountains excrete toxo that gets washed down in the watershed. It then moves through the food chain, getting into monk seals and cetaceans and other species.”


That’s a likely scenario for spinner dolphins, although with a possible twist. “We don’t know for sure,” says West. “It’s well accepted that [infected dolphins] are either eating contaminated food or they’re swimming around with their mouths open and drinking contaminated water.”


But West suggests another possibility: Absorption through the skin. “Don’t think of it as impenetrable to water.”


There’s much about toxoplasmosis that’s unknown.


The discovery of toxo in spinner dolphins spurred West to conduct retrospective tissue analyses on 22 spinner dolphins and 30 other stranded cetaceans representing 16 different species. “There was a case 26 years ago in which a spinner dolphin with toxoplasmosis died,” West says. “It makes us wonder how big a problem this is. Is this an odd case once every 25 years? Or is this much more prevalent and a threat to populations?”


What about bottlenose dolphins, melon-headed whales, striped dolphins and humpback whales? “If we’re talking about swimming through these waters and potentially ingesting oocysts in the water, there’s nothing to say humpbacks aren’t affected,” West says, even though scientists believe humpbacks don’t forage during their migrations through Hawai‘i’s warmer waters. “Ultimately, we want to see if toxoplasmosis is having an impact on other cetaceans besides spinners.”


Toxoplasmosis is not a Hawai‘i-only phenomenon. It’s widespread across the world. In New Zealand, tests on endangered Hector’s dolphins revealed 25 percent of the study group had died of toxoplasmosis and more showed signs of exposure. Like Hawai‘i’s spinner dolphins, these marine mammals spend a fair amount of time in near-shore waters.


To better understand the parasite, West’s study will also compare genotypes—that of the toxo found in L414 to known strains from Hawai‘i and elsewhere.


Results of her study are due this fall.


For single-celled organisms, these parasites are tricky.


Many people wonder whether a vaccine could be developed. But Dr. Thierry Work says that’s doubtful. It’s akin to trying to come up with a vaccine for malaria. “The parasite basically has the ability to shed its coat,” Work says. He’s a wildlife disease specialist with the U.S. Geological Survey’s National Wildlife Health Center in Honolulu. “At its simplest, the way the immune system recognizes a pathogen is by the color of its coat. But toxo has the ability to change its color, and thereby evade the immune response of its host.”


It’s not even a trivial matter to identify which cats have toxoplasmosis. A vet can test a cat for previous exposure to toxo, with a two-part blood test done on the Mainland, but a definitive diagnosis of illness can only be accomplished post mortem by looking for the protozoal organism in a cat’s tissues.


In a study similar to West’s, Work recently examined 300 nēnē carcasses. Hawai‘i’s state bird is also one of conservation’s hopeful stories. Reduced to fewer than 50 individuals in the 1940s, a captive breeding program helped boost the population to an estimated 2,500 today.


Work found as many as 40 percent of the 300 carcasses in his study were infected with toxoplasmosis. A much smaller 4 percent actually died of the disease. But, as we’re learning from toxo, that’s not enough to rule out the threat of the disease, which has killed other native bird species in Hawai‘i, including the koloa maoli, Hawaiian duck, Hawaiian moorhen, red-footed booby and the ‘alala, Hawaiian crow.


“A major cause of death in the study was due to trauma,” Work says, including vehicular strikes and predation. “But one wonders if these trauma birds died because they were infected with toxo.”


According to Work, a growing body of evidence is showing the toxo parasite has the ability to modulate the behavior of its host, even in healthy-looking animals, by reducing their fear and increasing more risk-taking behavior. Sly little parasites.


The cats don’t want to leave. They don’t try to escape, this is their home.” — Keoni Vaughn


On the westernmost tip of o‘ahu, where Hawaiian tradition says Hawaiians leapt from this world to the next to meet their ancestors, sits Ka‘ena State Park. It’s home to native plants, seabirds and sand dunes on which monk seals are known to haul out and rest. Here, a fence runs nearly a half mile, encompassing 64 acres, to keep out mammalian predators bigger than a 2-day-old mouse, including pigs, mongoose, rats, dogs and cats. This is where the Laysan albatrosses nest. Since the fence was installed, nest success jumped from 21 percent to nearly 80 percent. And the number of wedge-tailed shearwater chicks grew an astonishing 384 percent, from 614 to 2,359 within two years. Similar fences have been built on Kaua‘i and Hawai‘i Island. Clearly, protection works. But fences like this are costly to build and maintain. Plus, they only work with birds that nest in colonies, and do nothing to reduce the number of cats possibly infected with toxoplasmosis already on the landscape.


Elsewhere, some aggressive steps have been taken to alleviate the killing of native wildlife by non-native predators. New Zealand, known for its progressive conservation practices, announced earlier this year an ambitious plan titled “Predator Free New Zealand,” with the goal of eradicating all invasive mammals, including feral cats, by 2050. This came on the heels of Australia’s plan to cull up to 2 million feral cats by 2020. Hawai‘i doesn’t seem ready for such extreme action. A state bill to prohibit the feeding of “unrestrained predators” on state lands was deferred earlier this year.


But, on Kaua‘i, County Councilwoman JoAnn Yukimura is poised to introduce a bill to allow heavily managed TNR colonies on the island. “TNR” stands for “trap-neuter-release,” a controversial program that many conservationists say doesn’t work, and cat advocates say does. The idea is feral cats fed in a colony are spayed and/or neutered. If no new animals are added, over time the colonies naturally get smaller through attrition. Sounds great. But some say colonies simply invite increased abandonment so they not only don’t get smaller, they grow in size.


Under Yukimura’s bill, colonies would be licensed and required to follow strict parameters, including that no new cats be introduced to the licensed colonies. “They claim the goal is going to be attrition,” Yukimira says, “and so we’re saying, if that’s your claim, you have to demonstrate that.”


Yukimura’s bill doesn’t have the teeth of, say, New Zealand’s laws, but it does illustrate an effort to restore some balance to the ecosystem. It might be enough of an effort to stave off a lawsuit, the likes of which The American Bird Conservancy filed against the state of New York earlier this year, claiming a violation of the Endangered Species Act by allowing feral cat colonies near nesting colonies of threatened piping plovers.


Some attitudes are changing. Take Halford’s decision to screen her backyard. It aligns with the American Veterinarian Medical Association’s updated policy that suggests keeping cats indoors, supervised on a leash or in an outdoor enclosure.


Lana‘i cat sanctuary

Cats at the Lāna‘i Cat Sanctuary. Photo: Steve Czerniak



On Lāna‘i, where Perkins made his dire observations more than 100 years ago, Keoni Vaughn is executive director of the Lāna‘i Cat Sanctuary, a cage-free, open-air, fenced facility for lost, unwanted and abandoned cats. When the sanctuary recently expanded from 15,000 to 25,000 square feet, Vaughn had nightmares for weeks. He feared when the old fence went down the cats would somehow breach the new one, and he’d have 400 cats running amok. What happened? Nothing. “The cats don’t want to leave. They don’t try to escape,” Vaughn says. “This is their home.”


The sanctuary got its start when founder Kathy Carroll created a TNR program in 2004. That evolved into an actual fenced refuge for cats when a colony of endangered Hawaiian petrels was found atop the island’s tallest mountain.


“We value cats as much as we value birds,” Vaughn says. In fact, in response to criticism that the sanctuary allows adoptions, Vaughn says it will not permit adoption of any cats from near the petrel colony. Adopters of other cats agree to keep them indoors at all times or leave them at the sanctuary.


The saying, “There’s more than one way to skin a cat,” is horrible when taken literally. But, metaphorically, it’s dead on for the challenge of saving Hawai‘i’s endangered species from extinction. There will need to be numerous approaches. And Kristi West says it best. “Ultimately the problem is not going to be solved unless people decide it’s something we work at solving together.” That’s why Diane Halford now keeps her cats at home, where it’s safer for them and everyone else.



Toxo cat

Illustration: Michael Byers



How’d They Get Here?

No one’s sure exactly when cats arrived in the Hawai‘i, although most speculate it was the late 1700s or early 1800s.In 1840, explorer William Brackenridge reported feral cats in the wilderness around the volcano of Kīlauea. In 1866, Mark Twain riffed for the Sacramento Union about the numerous kinds of cats he spotted in Honolulu: “I saw cats—tomcats, Mary Ann cats, long-tailed cats, bob-tail cats, blind cats, one-eyed cats, wall-eyed cats, cross-eyed cats, gray cats, black cats, white cats, yellow cats, striped cats, spotted cats, tame cats, wild cats, singed cats, individual cats, groups of cats, platoons of cats, companies of cats, regiments of cats, armies of cats, multitudes of cats, millions of cats, and all of them sleek, fat, lazy and sound asleep.”



A few native species vulnerable to feral cats:



Hawaiian Stilt

Hawaiian Coot

‘Alae Ke‘oke‘o

Hawaiian Coot

Hawaiian Moorhen

‘Alae ‘Ula

Hawaiian Moorhen

O‘ahu ‘Amakihi

Hawaiian Honeycreeper

Ilio Holo I Ka Uaua

Hawaiian Monk Seal

Koa‘e Kea

White-Tailed Tropicbird


Pacific Golden Plover


White Fairy Tern


Laysan Albatross


Hawaiian Spinner Dolphin


Hawaiian Goose


Hawaiian Finch

‘Ua‘u Kani

Wedge-tailed Shearwater

Hawai‘i bird photos by Tom Dove, courtesy of the Hawai‘i Audubon Society. Learn more at hawaiiaudubon.org.
Photos: Hawaiian Spinner Dolphin, Courtesy of NOAA; Nēnē, Kelsey Ige