Domestic cats in the United States kill up to 3.7 billion birds and as many as 20.7 billion mice, voles and other small mammals each year, biologists estimated on Tuesday. Puss is probably the biggest human-induced killer of these species, outstripping better-known culprits such as habitat loss, agricultural chemicals or hunting, they said in a study published in the journal Nature Communications. A team led by Scott Loss at the Smithsonian Conservation Biology Institute in Washington looked at published research into the predation habits of cats. Cats that have outdoors access kill between 30 and 47 birds apiece in temperate parts of Europe and North America each year, and between 177 and 299 mammals, according to past investigations. The next step was to get an estimate of the number of cats in the United States. Loss\'s team calculated there were around 84 million cats with owners, of which a couple of million are unlikely to have outdoor access or go hunting. Added to that are between 30 and 80 million \"unowned\" cats -- animals that are wild or free-ranging but without an owner and survive on goodwill. \"We estimate that free-ranging domestic cats kill 1.4 to 3.7 billion birds and 6.9 to 20.7 billion mammals annually,\" says the study. \"Unowned cats, as opposed to owned pets, cause the majority of this mortality.\" The paper says the estimates are much bigger than previously thought, and show that cats \"are likely the single greatest source of anthropogenic [man-made] mortality for US birds and mammals.\" It adds: \"Scientifically sound conservation and policy intervention [are] needed to reduce this impact.\" The study tried to get a fix on the numbers of reptiles and amphibians that are killed by cats, but drew a blank. According to the famous \"Red List\" compiled by the/ International Union for Conservation of Nature (IUCN), cats on islands have caused or contributed to the extinctions of 33 species of birds, mammals and reptiles. The study coincides with a fierce debate in New Zealand, where Gareth Morgan, a businessman turned philanthropist, has called for cats there to be eradicated to save the country\'s unique species of wildlife, which includes the flightless kiwi.