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.
Seabirds are particularly vulnerable to cats because they nest on the ground.
Most seabirds now nest on offshore islands, where cats and mongooses can't get them. 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. 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.