So what are we gonna do about the feline question? Cats are killing upwards of 1 billion birds in the United States alone on the low end of the estimate. The ecological impact is unquestionable. All of the birds should be eating hundreds of insects annually. No doubt if there were more birds around we wouldn't have to worry so much about our food being poisoned with insecticides. The birds are nature's insecticides. In communist China they attempted to eradicate sparrows because they thought that the sparrows were eating too much of their rice crop. What actually happened was that locust swarms, no longer facing predation ran rampant and caused one of the largest famined in recorded history.

http://www.usatoday.com/story/news/nation/2013/01/29/cats-wild-birds-mammals-study/1873871/

https://en.m.wikipedia.org/wiki/Four_Pests_Campaign