The Big Four’s strategy for the warm-weather months in recent years has been to bring back popular reality shows and throw out a bunch of other new ones against the wall.

But so far — even amid a summer of record-setting heat and humidity across the country — nothing’s sticking.

Add in the fact that some reality stalwarts have been unable to keep up with their year-ago pace and repeats continue to deliver diminishing ratings returns in the DVR era, and the broadcasters have lost a meaningful chunk of their audience.

According to Nielsen estimates for the early summer (May 28-July 8), ABC, CBS, NBC and Fox have combined to average a 6.0 rating/19 share — down 10% vs. last year (6.7/21). During the same stretch, basic cable is up 3% (17.9/57 vs. 17.3/55), meaning that it now commands 75% of summer, ad-supported viewing among adults under 50 (up from 72% last year).

The Big Four are faring a bit better in total viewers, down 8% vs. last year while basic cable is up 1%.

NBC’s “America’s Got Talent” remains TV’s top summer attraction, though it’s down some from its especially hot pace of last year when it aired alongside “The Voice.” ABC’s “The Bachelorette” and “Wipeout” are down modestly.

One of the stronger performers of summer is Fox’s “MasterChef,” which, now paired with “Hell’s Kitchen,” is up more than 15% vs. last year. The net’s “So You Think You Can Dance” is flat or up slightly in most categories.

Certainly, the networks haven’t gotten much help from young reality shows this summer.

Some newbies have struggled from the start, including ABC’s “The Glass House,” “Final Witness” and “Duets” and CBS’ “Dogs in the City.” (CW has also struck out with its two tyros, “Breaking Pointe” and “The Catalina.”) And others — like NBC’s “American Ninja Warrior” (which had aired on cabler G4) and Fox’s dating duo “Take Me Out” and “The Choice” — have fared just decently.

Also in that passable category are a few shows that had aired for a short time before (ABC’s “Secret Millionaire” and “Extreme Makeover: Weight Loss Edition” and NBC’s “Love in the Wild”).

On the scripted side, ABC’s Canadian import “Rookie Blue” is holding up OK in its third summer (on par with last year), while the only other new entry, NBC’s “Saving Hope,” is down from the net’s scripted original of last summer, “Love It wouldn’t be surprising to see the nets try more scripted originals next summer, at least ones that make sense financially; CBS has already announced plans to bring back crime drama “Unforgettable” for a batch of summer episodes in 2013.

From a network standpoint, ABC is the narrow leader in adults 18-49 (1.7/6) but is down 15% due to a longer NBA Finals a year ago. Fox and NBC are tied for second (1.6/5), with the former flat year-to-year and the latter down 15% from 2011, when the first season of “The Voice” extended through June.

CBS is a distant fourth (1.1/3), down 8%, as the net traditionally struggles in early summer before “Big Brother” heats things up. It has been buoyed by the amazing “The Big Bang Theory,” whose repeats are up 21% in 18-49 vs. a year ago.

Among all viewers this summer, leader CBS (5.7 million) is slightly ahead of NBC and ABC (both 5.6 million).

Univision and Fox tied for the lead among adults 18-49 for the week of July 2-8 (1.3/4), with the former pulling ahead outright in the younger 18-34 half of the demo. They were followed in the 18-49 rankings by NBC (1.2/4), ABC (1.1/4) and CBS (0.9/3).

NBC eked ahead in adults 25-54 (1.6/5 to 1.5/5 for Fox), and CBS led in total viewers (5.3 million).

Fox was led by two nights of cooking shows “Hell’s Kitchen” and “MasterChef,” while Univision was paced by “La Que No Podia Amar” (The One Who Couldn’t Love), which led the 10 p.m. hour on a Monday-Friday basis.

NBC’s “America’s Got Talent” was the No. 1 show overall once again (3.0/9, 10.40m for Monday’s telecast), and the net also ruled on July Fourth with the annual “Macy’s 4th of July Fireworks Spectacular” (1.8/7, 8.12m).

ABC’s top entry was “The Bachelorette” (2.5/8, 7.49m), which is heating up as it nears its finale.

TNT held off USA to lead the cable networks in all categories. The Turner net had three of the week’s four most-watched dramas on cable (“Rizzoli & Isles,” “Falling Skies” and “Dallas”).

In 18-49, USA’s Monday vet “WWE Raw” (1.7/5 in 18-49, 4.70m) tied with the same night’s “Pawn Stars” on History (1.7/5, 4.94m) as basic cable’s top shows of the frame.

FX’s “Anger Management” (1.3/4, 3.37m) dropped off about 40% from its strong premiere but was the top scripted basic-cable program of the week in 18-49.

HBO’s “The Newsroom” drew its largest audience to date Sunday (0.9/2 in 18-49, 2.21m). Lead-in “True Blood” held steady (2.6/8 in 18-49, 4.50m) and was the No. 2 primetime program of the week among young adults.