U.S. Department of the Interior

U.S. Geological Survey

USGS FS 2006-3006

January 2006 Printed on recycled paper

Background

Domestic cats (Felis catus) were first brought to Hawai‘i 

aboard sailing ships of European explorers and colonists. The 

job of these predators was to control mice and rats on the ships 

during the long voyages. As in other places, cats were taken in 

and adopted by the families of Hawai‘i and soon became household pets known as popoki. But cats have always been very well 

equipped to live and hunt on their own. On tropical archipelagos 

like the Hawaiian Islands where no other predatory mammals of 

comparable size existed, abundant and naïve prey were particularly easy game, and cats soon thrived in the wild.

Although the details of when cats first came to live in the 

wild remain little known, adventurers, writers, and naturalists 

of the day recorded some important observations. Feral cats 

were observed in remote wilderness around Kˉılauea volcano 

on Hawai‘i Island as early as 1840 by explorer William Brackenridge. Mark Twain was so impressed by the great abundance 

of cats when he visited Honolulu in 1866 that he reported his 

observations in the Sacramento Union newspaper, which were 

later reprinted in his book Roughing It:

I saw... 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...

Cats Prey on Native Birds in a Variety of Habitats

Cats were also common in remote and isolated communities, 

as noted by the famous traveler, Isabella Bird, during her visit to 

Waimanu Valley on Hawai‘i Island in 1873. By that time, a disturbing but little known phenomenon was beginning to happen 

to the unique birds of Hawai‘i. At least 30 species or subspecies 

of endemic forest birds were either greatly reduced in number 

or became extinct between 1870 and 1930. One of the earliest naturalists to implicate feral cats as a causal factor in these 

losses was R.C.L. Perkins, who wrote in 1903:

On Lanai, in walking up a single ravine, I counted the remains of no less than twenty-two native birds killed by cats, 

and these must all have been destroyed within two days, as 

previously the whole gulch had been washed out by a heavy 

flood. Two cats were actually shot on this occasion as they 

were devouring their prey, and several others seen, but, owing 

to the fact that they are extremely shy and mostly nocturnal in 

habits, few people who have not lived much in the woods have 

any idea of their numbers. 

In Hawai‘i today, cats range from relatively high densities near sea level where abandoned pets are frequently fed by 

well-meaning animal-lovers, to sparse isolated populations in 

remote rain forests and even alpine areas on the islands of Maui 

and Hawai‘i. Cats kill songbirds, which nest, feed, and roost in 

trees, and ground nesting birds, such as quail and other introduced game birds. Native sea birds and other species that nest 

on the ground or in burrows are particularly vulnerable to predation by cats. For example, endangered ‘ua‘u (Hawaiian Petrel; 

Pterodroma sandwichensis) are particularly vulnerable because 

they raise their single chicks in lava crevices and nestlings cannot fly for more than 15 weeks after hatching. Adult ‘ua‘u are 

also often killed by cats, causing serious population declines 

because it takes each bird five to six years to reach 

Feral Cats: Too Long a Threat to Hawaiian Wildlife 

This feral cat was photographed by a remote camera in Hawai‘i Volcanoes National Park (see page 3). 

Left. ‘Ua‘u are rare and endangered long-lived 

seabirds that feed on fish and squid offshore but nest in 

lava crevices at high elevation on volcanoes. (Photo by W. Banko) Right.

This adult ‘ua‘u was killed by a cat high on the slopes of Mauna Loa in 

Hawai‘i Volcanoes National Park. (Photo by F. R. Warshauer, USGS)
feral cats 

using various tools including remote cameras and telemetry.

Remote Cameras Capture Images of Feral Cats

Surveillance cameras installed at nests of the palila revealed 

that cats reduce palila nest success. Cats mainly kill palila

chicks, although adult females may be killed while sitting on 

the nest. Cat kills at nests not being monitored by cameras are 

identified by hair, claw marks, and tracks around the nest. Up 

to 11% of palila nests are depredated by feral cats each year. 

These losses may threaten the survival of the species, which 

lays few eggs each year and takes a long time to develop to 

adulthood. In contrast, mainland songbirds have much larger 

clutches and raise nestlings much more quickly, decreasing 

maturity. Even birds that nest and feed in wetlands are 

killed by cats, demonstrating that no bird in any habitat is 

safe from these stealthy, wide-ranging, introduced predators.

Cats Carry and Transmit Diseases to Other Wildlife

Not only do cats prey on vulnerable endangered birds, but 

they also carry diseases that may infect endangered birds and 

other wildlife. Cats are the definitive host of a potentially fatal 

disease called toxoplasmosis. In Hawai‘i, toxoplasmosis has 

killed native Hawaiian birds such as the critically endangered 

‘alalˉa (Corvus hawaiiensis), the endangered neˉneˉ (Hawaiian 

goose; Branta sandvicensis), and even seabirds such as the 

red-footed booby (Sula sula). Because the protozoan organism 

that causes toxoplasmosis (Toxoplasma gondii) can complete 

an important part of its life cycle in seawater, this disease also 

poses a threat to marine mammals. Fatal toxoplasmosis was 

recently confirmed in an endangered Hawaiian monk seal (Monachus schauinslandi), and may possibly threaten other marine 

mammals in Hawai‘i such as the spinner dolphin (Stenella longirostris). In addition to threatening wildlife, toxoplasmosis is 

zoonotic and poses a significant health risk to pregnant women. 

Feral cats are also known to carry bacteria that have killed pueo

(Hawaiian Short-eared Owls; Asio flammeus sandwichensis) 

and ‘alalˉa.

USGS Studies Ecology of Feral Cats in Subalpine 

Hawai‘i Island

Feral cats now live throughout the dry subalpine and alpine 

shrublands of Mauna Kea and Mauna Loa on the island of 

Hawai‘i. Being completely isolated from human settlements, 

these cats live entirely off the land under extreme conditions, 

enduring freezing temperatures, battling lethal diseases, and 

relying entirely on hunting wildlife. Although feral cats exist 

in small numbers in these rugged ecosystems, they can be very 

damaging to endangered birds, such as the palila (Loxioides 

bailleui) and the ‘ua‘u. Understanding how feral cats live, 

move, and feed will allow us to formulate better strategies to 

protect endangered birds. The USGS has been investigating 

The death of adults has very severe consequences for populations 

of long-lived birds, because their reproductive rates are low and 

parental investment in raising young is high. (Photo by J. Jeffrey)

Feral cats are difficult to study because of their nocturnal habits and 

wariness of people. Infrared-triggered cameras are used to determine 

the presence of cats in the vicinity of endangered birds. 

This female n ̄en ̄e was killed by a cat while sitting on her nest in the Ka‘u 

Desert on K ̄ılauea volcano in Hawai‘i Volcanoes National Park. Cat kills 

are identified by cat hair, claw marks and tracks around the remains. 

(Photo by Kathleen Misajon, NPS)
threaten 

nesting birds 

from great distances. In 

addition, several cats may hunt 

in the same area, exposing birds to threats 

from a host of different cats over a short period of 

time. In the case of palila occupying only about 140 km2

(54 mi2), the home range of each male cat represents more than 

10% of this area. Cats are most active at night, when birds have 

settled on their nests and are particularly vulnerable.

Diet

Stomach contents from feral cats captured at two sites were 

examined in order to characterize the diet of these introduced 

predators (Figure 1). Feral cats on Mauna Kea subsist to a great 

extent on invertebrates and birds, including both songbirds 

their exposure to predators dramatically. In addition, many 

Hawaiian birds exhibit little fear of potential predators and often 

lack avoidance behaviors. 

Telemetry

Radio transmitters were attached to a sample of cats captured 

on Mauna Kea in order to describe their home ranges, movements and activity patterns. 

On Mauna Kea, some male 

cats can range up to 20 

km2 (8 mi2) and this rivals 

the largest reported home 

range for a feral cat from 

anywhere in the world. In 

contrast, the home ranges 

of three females averaged 

less than 8 km2 (3 mi2).

The significance of these 

large home ranges and daily 

movements is that cats 

Palila chicks are highly vulnerable to predators because their nestling 

development period is approximately twice as long as that of continental songbirds.

In order to track the movements of feral cats in endangered palila territory, cats were trapped and fitted with radio collars before release back 

to the wild. 

The home ranges of a male (orange) and a female (blue) feral cat within 

the critical habitat of the endangered palila (red lines) on the West 

Slope of Mauna Kea. The home range of the male was 20.1 km2

, and the 

home range of the female was 6.1 km2

. Smaller shapes inside the home 

ranges are areas that the cats used more intensively. 

This palila nest was discovered on Mauna Kea (Hawai‘i Island) after the 

chicks had been killed by a cat. Predation can have significant impacts 

on the population of this endangered bird.
and game birds. Some stomachs contained leg bands from 

both native and introduced songbirds that had previously been 

captured and banded at the site. Insects are also common prey 

items. Surprisingly, rodents are less important than birds and 

insects despite the great abundance of mice in this environment. 

In contrast, in Hawai‘i Volcanoes National Park (HAVO), the 

most common prey items are mice and invertebrates, including 

grasshoppers and flies. While no gamebirds were found; one 

stomach contained remains of a young ‘ua‘u, identified by a 

complete mandible set, feathers, and bones. Most of the rodent 

remains were mice; rats were rarely found. The diet of cats in 

HAVO suggests that cats may subsist on common 

prey such as mice and insects, but then 

turn to birds when they are more 

accessible during nesting periods.

Conclusions

Since their introduction to Hawai‘i in 

the 1800s, cats have adapted to life on their own in 

Hawaiian ecosystems, where they are now abundant. Their success in these habitats has been at the expense of native animals, 

which evolved in the absence of mammalian predators. Native 

birds are particularly vulnerable. In addition to predation, cats 

also impact native wildlife by transmitting lethal diseases. For 

the ‘alalˉa, neˉneˉ, palila, and ‘ua‘u, unnatural predation, in addition to other factors, continues to threaten the survival of these 

endangered species. Given enough time, feral cats may even 

cause the extirpation of some populations of these birds. USGS 

is currently collaborating with NPS to reduce the threat of these 

introduced predators to endangered species in parks throughout 

Hawai‘i. These findings will help protect native birds whether 

they occur in areas managed by USFWS, State of Hawai‘i, or 

on privately-owned lands.

Suggested Reading

Banko, P.C. et al. 2002. Availability of food resources, distribution of invasive species, and conservation of a Hawaiian 

bird along a gradient of elevation. Journal of Biogeography 

29: 789–808.

Perkins, R.C.L. 1903. Vertebrata (Aves). Pp. 368-465 in Fauna 

Hawaiiensis, Vol. I, part IV (D. Sharpe, ed.). The University Press, Cambridge, England.

Simons, T.R., and C.N. Hodges. 1998. Dark-rumped Petrel 

(Pterodroma phaeopygia). Birds of North America, No. 

345. (A. Poole and F. Gill, eds.). The Birds of North 

America, Inc. Philadelphia, PA.

Tomich, P.Q. 1986. Mammals in Hawaii, 2nd Ed. Bishop Museum Press, Honolulu, HI.

Winter, L. 2003. Popoki and Hawai‘i’s native birds. 

‘Elepaio 63: 43–46.

For more information contact:

Station Leader, Kilauea Field Station 

Email: PACUC@usgs.gov 

Fact sheet written by:

Steven C. Hess and Paul C. Banko, 

USGS Pacific Island Ecosystems Research Center 

Photo credits:

Background image this page by Rex Williams. Other photos by USGS, 

except where noted.

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Figure 1. Stomach contents of cats captured at two subalpine sites on 

Hawai‘i Island. 

What the Public Can Do

• Be a responsible pet owner by keeping your 

cat indoors as much as possible, especially at 

night; keeping track of its whereabouts; and 

making sure your cat is well-fed at all times.

• Cats that are properly cared-for and kept 

indoors have been shown to live longer, 

healthier lives than do feral cats. Please do 

not release cats into the wild because it is 

inhumane to both the cats and to the wildlife 

on which they prey.

• Please help to reduce the population of feral 

cats by taking them to the Humane Society 

rather than feeding them. Feeding feral animals only increases their numbers. 

• Have your cats spayed or neutered. ”If you 

feed it, fix it.”

A complete mandible of a young ‘ua‘u was found in a feral cat stomach 

from Mauna Loa in Hawai‘i Volcanoes National Park. 

1 cm