Our National Cat Problem

From a New York Times article passing along the startling news that cats hunt and kill birds:

The American Bird Conservancy estimates that up to 500 million birds are killed each year by cats — about half by pets and half by feral felines. “I hope we can now stop minimizing and trivializing the impacts that outdoor cats have on the environment and start addressing the serious problem of cat predation,” said Darin Schroeder, the group’s vice president for conservation advocacy.

As it happens, I've been trivializing the impact of cats on birds my entire life. I mean, cats kill birds. Bears kill salmon. Birds kill worms. The strong survive and the adaptive fitness of the species is improved. Cycle of life and all that. Why are we supposed to be upset about this?

But despite the fact that my own cats contribute 0% toward this avian holocaust, I understand that I have biases in this area. Maybe I'm taking things too lightly. So here's my question: assuming that this 500 million number is correct, what percentage of the entire bird population does this represent? I did a bit of desultory googling but got bored before I found an answer. So I'll put my vast audience to work. How many birds are there in the United States? Do cats kill 10% of them each year? 1%? A tenth of a percent? Just how serious is this national scourge of cat predation?