There is an interesting article in the science section of the New York Times about the effects of domestic cats on bird populations.
As the headline suggests, domestic cats are a major threat to bird populations. The article suggests that over 500 million birds are killed by cats each year in the United States-half by feral cats and half by house pets. The article also makes an interesting point that people get all upset about bird deaths due to wind turbines and yet seem to be much less upset about bird deaths due to cats. Wind turbines by the way can apparently be situated to minimize bird deaths.
We accept predation as somehow natural but forget that the domestic cat is not native to North America and so is really an invasive species-so they are more like those big constrictor snakes that people have let loose in the everglades. Now if only we could train cats to attack non native birds such as starlings or English sparrows but I don't think that is going to happen.
Just by chance, my indoor only cat Carl caught and killed a chickadee that had gotten on to our screened in back porch yesterday. He had caught one other chickadee during the winter but he just held it in his mouth very gently and I managed to rescue the bird. This time he clearly figured out that the next step in hunting after catching the bird is to kill it.
I hope he never figures out the eating stage.