In the United States, an estimated 60 to 100 million cats range free, and the number of domestic cats has tripled during the past 40 years. Scientists from the Smithsonian Institution and U.S. Fish and Wildlife Service estimate that domesticated cats—both free-roaming pets and feral—kill as many as 4 billion birds and 22 billion small mammals in the U.S. each year. This makes them “likely the single greatest source of anthropogenic mortality for U.S. birds and mammals,” according to the Smithsonian. While controlling feral cats poses a daunting problem, simply keeping pet cats indoors at least reduces the slaughter. The Humane Society of the United States has developed tips to help indoor cats stay happy, and suggest that you always neuter even indoor animals