By Laura Zuckerman Jan 29 (Reuters) - Think of cats as cute purring bundles of fur? Think again. A new study says free-roaming kitties are serious killers. Such cats are a leading cause of deaths of birds and small mammals in the United States, with pet and ownerless cats blamed for killing up to 3.7 billion birds and as many as 20.7 billion other animals each year, government scientists said in a study released on Tuesday.