colony was surrounded by ultrasound generators intended to deter cats, the fairy tern colony might have been an irresistible target, and this particular cat was white and had a blue eye, traits commonly associated with deafness. Effects on wildlife In the United States, free-ranging cats kill one to four billion birds and six to 22 billion mammals annually. In Australia, domestic cats were introduced in the 1800s to settlements that had developed near gold mining sites and farms as a pest control strategy to decimate rabbits, mice, and rats. Feral cats kill on average one million reptiles each day. It has been estimated that they kill more than 800 million mammals annually, of which 56 percent are native species. Impact on prey species To date, little scientific data is available to assess the impact of cat predation on prey populations outside of agricultural situations. Even well-fed domestic cats may hunt and kill, mainly catching small mammals, but also birds, reptiles, amphibians, fish, and invertebrates. Hunting by domestic cats may be contributing to the decline in the numbers of birds in urban areas, although the importance of this effect remains controversial. Feral cats can threaten native species with extinction. Controlling or eliminating the populations of the non-native cats can produce a rapid recovery in native animals. Native species such as the New Zealand kakapo and the Australian bettong tend to be more ecologically vulnerable and behaviorally "naive", when faced with predation by cats. Feral cats have had a major impact on these native species and have played a leading role in the endangerment and extinction of many species. In Hawaii's remote, mountainous areas, they destroy the nests of seabirds including Newell's shearwater (Puffinus newelli) and the Hawaiian petrel (Pterodroma sandwichensis). In agricultural settings, cats can be effective at keeping mouse and rat populations low, but only if rodent harborage locations (such as tall grass) are kept under control. While cats are effective at preventing rodent population explosions, they are not effective for eliminating pre-existing severe infestations. In systems where wildlife is threatened by both predation by rats and cats, there are concerns that controlling cats could increase predation by rats, due to rat populations increasing. For example, in Christmas Island, it was shown that decreasing cat populations would improve the growth rate of a threatened bird, as long as rats did not increase by more than 77 rats per cat removed.