Actually, the data show that it is "believed" that out of 15-20 billion birds landing in the US each year, 2.3 billion may be the victims of the 80 million feral or 84 million "owned" cats (probably a lot less of the latter, given that a lot apprently are permanently grounded). So, in other words, 13.1% (11.5-15.3%) of birds fall prey to cats. While that does indeed fit within the range of the actual Roman definition of "decimation," it ain't likely to be a game-changer. As has been pointed out elsewhere, many victim birds will be weak (through injury, illness, or age), and of course many will be species that are either not even remotely threatened, or may be regarded as pests, anyway (e.g. pigeons), and won't be going the way of the Stephens Island Wren.
Of course, it could also be said that the domestic dog did not evolve in the United States, either.