TVs are quickly becoming viewers’ second screen — a phenomenon made crystal-clear as networks aired their season finales last month.

The live ratings for the shows — across all networks tumbled 30 and 40 percent from a year ago as viewers more and more turned to their mobile devices for entertainment.

The the result are coming at a very inopportune time: as networks begin to talk their book to Madison Avenue, which is about to place some $70 billion in TV ad commitments.

Only six of 61 returning network show finale episodes saw an uptick in ratings versus the prior year, according to Nielsen numbers for live plus same-day TV viewing in the 18-to-49-year-old category.

Everywhere else, it can only be described as a bloodbath.

“Quantico,” starring Priyanka Chopra, is ABC’s biggest loser in the demo category, off some 43.3 percent versus last year’s season-finale episode. The show has lost viewers, in part, because its time slot has shifted.

At CBS, the finale of the per­ennially popular “Big Bang Theory” fell 19.2 percent in the demo — and it wasn’t the network’s biggest loser.

That distinction belongs to “NCIS” — now absent actor Michael Weatherly, who played Anthony DiNozzo — which lost 43.6 percent of its audience compared to a year ago.

The networks are waiting for the delayed viewing numbers, called C3, which add back in recorded playbacks of shows within a three-day period. They will boost ratings next week, but live ratings remain the most immediate gauge of who’s winning the time periods.

“There’s a real reason people pay a fortune to be in live sports, and why networks are doing live musicals. Advertisers want live. You are watching, and you are engaged,” said Billie Gold, vice president director of programming at ad agency Amplifi.

The CW show “Reign” saw its finale fall 30.8 percent. Gold says it had a better lead-in last year — part of the reason for the decline in the end-of-season rating.

The network’s “Vampire Diaries” grew, perhaps, because of a March finish versus May during the prior season.

“Empire,” Fox’s big hit, is also losing steam, declining 39.1 percent versus the year-ago finale number.

“Rosewood” fell 59.7 percent. Fox did see a few finales improve on their last year, including “Bones,” and Gordon Ramsay’s “Hell’s Kitchen.”

NBC’s finales also got lumped up, with Jennifer Lopez’ “Shades of Blue” tumbling 35.5 percent versus a 2016 March finale — perhaps contributing to the lower 2017 ratings. The network’s “Chicago Med” fell 30.7 percent.

Media consultant Brad Adgate says TV news continues to be a problem for entertainment ratings. “The news continues to be unrelenting in its ability to attract viewer’s attention; I think that’s hurting them,” he said.

In a report out June 2, Barclays media analyst Kannan Venkateshwar noted that the average person watched 2.5 hours on a smartphone in the fourth quarter — up from 1.3 hours a year earlier. That compared with 4.9 hours on TV.

For viewers in the demo, the smartphone was more popular than TV, something he called a “considerable shift” in consumption patterns.