Of course we are, as much as we are responsible for the damage cats do. There are no domestic cats without humans.

But what is the purpose of making a comparison between bird deaths from building strikes and bird deaths from cat predation? Should we throw up our hands, "Oh, since birds die from flying into buildings I guess there's no point in keeping my cat inside." Seriously, are you just trying to obfuscate the point, or are you interested in moving the discussion forward?