"Previous studies had suggested that cats kill about 500 million birds a year. Marra's group came up with something very different. "We estimate that cats kill somewhere between 1.4 and 3.7 billion birds a year," Marra says. "For mammals, it's upward of about 15 billion."
"Marra says based on those new figures, cat-caused mortality far exceeds deaths from other sources, like collisions with cars or wind turbines.
http://www.npr.org/2013/01/29/170588511/killer-kitties-cats-...
"Previous studies had suggested that cats kill about 500 million birds a year. Marra's group came up with something very different. "We estimate that cats kill somewhere between 1.4 and 3.7 billion birds a year," Marra says. "For mammals, it's upward of about 15 billion."
"Marra says based on those new figures, cat-caused mortality far exceeds deaths from other sources, like collisions with cars or wind turbines.