I took my dead yard and added about a dozen apple, fig, and shade trees. This brought some birds for sure. I would still expect to see more though given the seed I put out.

I'm guessing massive cities and suburban sprawl are a big part of the problem. Most yards are literally just grass which often requires tons of weed killer and fertilizer in some areas. As a society we're so dumb sometimes. My mother also has a bunch of cats and they've killed dozens of birds over the years. That probably adds up.