Once considered immune to the audience erosion plaguing the television industry, ratings for the National Football League have slipped through the first four weeks of the season.

TV networks have spent heavily on sports, and the NFL in particular, because of their must-see nature. While more viewers today watch commercial-free streaming services like Netflix or record shows on DVRs and skip the ads, sports programming primarily is still watched live, making it valuable to advertisers.

Combined, ESPN, Fox, NBC and CBS are spending an average of $5 billion a year for football rights through 2021. The games not only score big ratings and ad sales, but are crucial platforms to promote other programming.

This season, network viewership is down about 10% from last season, according to Nielsen data, with steeper declines for prime-time games on Sunday, Monday and Thursday. The drop has caught advertisers and rights holders off guard and left them scrambling to find a cause.