A new study by NC State University shows that hunting by house cats can have big effects on local animal populations because they kill more prey, in a given area, than similar-sized wild predators. The researchers put GPS tags on around 1000 house cats for the study. Based on results, the study estimates that house cats in North America kill 10-30 billion small animals a year, with each house cat killing up to 39 animals each per year. So, if you like the songs of small birds or the sight of baby bunnies frolicking in the yard, keep your cat inside.