House cats have long been known to cause problems, e.g.

"The estimated kill rates are two to four times higher than mortality figures previously bandied about, and position the domestic cat as one of the single greatest human-linked threats to wildlife in the nation."

There is a Times article out which presents some evidence that the issues are worse than people thought:

"In a report that scaled up local surveys and pilot studies to national dimensions, scientists from the Smithsonian Conservation Biology Institute and the Fish and Wildlife Service estimated that domestic cats in the United States — both the pet Fluffies that spend part of the day outdoors and the unnamed strays and ferals that never leave it — kill a median of 2.4 billion birds and 12.3 billion mammals a year, most of them native mammals like shrews, chipmunks and voles rather than introduced pests like the Norway rat. "

* Article: That Cuddly Kitty Is Deadlier Than You Think