Actually - and not too surprising - it's humans who are the
top-killers of birds. Between building-strikes, impacts with cars,
pollution, deforestation (and other forms of habitat destruction),
pesticides and more, humans kill the most. (Yay?) However,
human-causes tend to be divied up into various sub-categories
(high-tension line collisions cause approx. 300 million bird deaths
alone), which helps disguise the impact.
But kitties are a close second (450 million a year).
Or, rather, third. Disease by far kills more birds (and, in general,
more animals) than anything else. A fact I think that, three years
ago, would have surprised a lot of people but recent events have
probably made more people aware of the sobering danger of microbial
(They're not wild critters so don't really count in the statistics
above, but 50 billion chickens are killed annually.)