@Vivian The article I read had this as the last paragraph:
Rosenberg's study didn't go into what's making wild birds dwindle away, but he pointed to past studies that blame habitat loss, cats and windows.
In my neighborhood, where I've been living for over 65 years, I would say it's loss of habitat. Our neighborhood has gone from rural to suburban over the course of the years. More paving, building, cutting of trees, loss of fields. But, I don't know the answer. People have to go somewhere. Not sure about cats and windows, but we had a cat when I was a kid that could bring in birds, phesants and even snakes!