The headline contradicts the findings of the study, so you missed this in the article: "The estimate puts windows behind only cats as the largest source of human-related menaces that kill birds directly." The article  from Science News linked below is itself linked in the WaPo piece you posted. Statistics infuriate me less than the inability of most newspapers to explain scientific topics clearly. 

https://www.sciencenews.org/article/cats-kill-more-one-billion-birds-each-year