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Volcanowatch

September 21, 2000

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.


Does Your Cat Eat Birds?

Cats have been our pets and rat-catchers since the dawn of civilization. Many of us grew up with cats or now keep them (including the authors of this column!). However, the number of domestic cats in the U.S. has more than doubled in the past 30 years, and they are becoming an increasing problem for wildlife. It's important to point out that we're talking about two sorts of cats, those that belong to someone vs. the homeless wild, or feral, ones.

Domestic cats were introduced to Hawai`i probably during the late 1700s, when sailing ships began regularly visiting the islands. Shipboard felines, brought along to keep the vessels clean of rodents, were given away or traded in port. By the 1800s, Mark Twain, for instance, noted that cats abounded in the islands. Contemporary naturalists also took notice and reported that cats were killing birds in the forest. Without cold winters to keep them home, cats had set up a life of their own far from the comforts of human care.

Cats are highly adaptable predators capable of killing a variety of animals and birds, and feral cats now inhabit every town, rural area, and forest in the islands, from the seashore to above timberline on the highest volcanoes. Here they encounter native birds that evolved without mammalian predators, and these birds are easy prey for cats. As early as 1903, naturalist R.C.L. Perkins noted serious depredation of native birds on the island of Lana`i. Today, biologists studying native forest birds continue to document predation of nesting adults and chicks by feral cats high on the slopes of Mauna Kea, Mauna Loa, Haleakala, and in the Alaka`i swamp.

Seabirds are particularly vulnerable to cats because they nest on the ground. The last nesting colony of Dark-Rumped Petrels (Ua`u,) on the Big Island is near the 9,000-foot elevation on Mauna Loa, and even at these barren, chilly heights, predation of adult Ua`u by cats is a serious problem. Most seabirds now nest on offshore islands, where cats and mongooses can't get them.

It's common knowledge that cats roam, but a recent study of radio-collared feral cats on Mauna Kea documented an average home range of 13.9 square miles! Average daily movement was about half a mile per day. One cat moved 25 miles within a 23-day period. An analysis of the stomach contents of 37 feral cats from Mauna Kea reported a diet of birds, insects, mice, and rats. Birds were found in 78% of the samples, and small songbirds were more than twice as common as game birds.

Recently, some people have advocated supporting managed cat colonies in public places to control rodent populations, but they have not considered the impacts these colonies can have on our birds. Most cats will wander and hunt whether they are fed or not, and cats don't eat just rats and mice. One cat from a managed colony could be responsible for repeated killings in a seabird colony or eating an entire brood of endangered Hawaiian Stilts in our wetlands. Feral cats also carry diseases, like toxoplasmosis, that can harm humans as well as kill birds.

It is costly and time-consuming to remove cats from remote areas, yet this may be one of the best ways to protect some seabird colonies. Our native birds face many problems in addition to predators. Habitat loss and avian disease have also contributed to the decline of native birds. Birds are an important part of the ecosystem and lift our senses with their song in our neighborhoods or on hikes in the forest.

Wild cats, and pet cats that are allowed to roam free, can have serious impacts on our bird populations. Some of the things that you can do to limit their access to birds include keeping your cat indoors (for more information, see www.abcbirds.com), having your cat spayed or neutered, and taking stray cats to your local humane society.

Eruption Update

Eruptive activity of Kilauea Volcano continued unabated during the past week. Lava is erupting from Pu`u `O`o and flowing through the old tube system for 1.5 km (0.9 mi) to the southeast. Breakouts from the old tube system at the 2,300-ft elevation feed a flow that has advanced down Pulama pali to the coastal flats where it is slowly wending its way to the sea. The flow is almost completely tubed over along its entire length, but molten material can be seen at the leading edge, which is within 150 meters (500 ft) of the coastline at Kamokuna.

There were no earthquakes reported felt during the past week ending on September 21.


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Updated: October 3, 2000