I believe the royal society for the protection of birds studied whether cats are a threat to bird populations and concluded that they really aren't. Obviously cats do kill birds, but overwhelmingly the major threat to birds is habitat loss caused by humans. Also, cats kill rodents, which indirectly helps birds because rodents are a big threat to bird populations, because rats take eggs from their nests. In fact, cats preferentially kill rodents. Something like 90% of their diet will typically be rodents. Birds, for them, are only opportunity kills. In other words, cats are an easy scapegoat because they quite visibly do kill birds, but humans (as is usually the case) are the true underlying problem.