Similar research has been done in the UK (I believe by the Mammal Society, sorry cannot find a reference) and the estimated number of bird deaths per year was 3

Similar research has been done in the UK (I believe by the Mammal Society, sorry cannot find a reference) and the estimated number of bird deaths per year was 300 million. This is a tough problem to quantify - typically the studies follow a number of cats closely and then extrapolate based on the estimated number of cats. Clearly this is close to an educated guess however the important part is that we are getting some idea of the scale of the problem. Given all the pressures on wildlife, this one is probably one of the more solvable problems.