Fox captured Wednesday’s ratings race with its two-hour installment of ”American Idol,” but the vet declined about 15% week to week for one of its lowest scores ever. A consistent CBS ran second on the night, while ABC’s ”Modern Family” was the top scripted program in demos.

According to preliminary national estimates from Nielsen, ”American Idol” averaged a 5.1 rating/14 share in adults 18-49 and 15.9 million viewers overall , on par with its two-hour average of last Thursday. But excluding Thursdays (a night it moved to last year), Wednesday’s two-hour ”Idol” is the lowest-rated, in-season regular episode of the show to date. On the same night a year ago, ”Idol” did a 7.9 rating in 18-49, and in 2010 it did a 9.0.

Still, ”Idol” won all four half-hours in 18-49, peaking with a 5.4/14 in the 9:30 p.m. half-hour, when it drew 16.8 million viewers overall.

Competing networks weren’t fully able to take advantage of ”Idol’s” declines, as spring-like rating declines have hit the broadcasters a few weeks early this year.

ABC’s ”Modern Family” (4.7/12 in 18-49, 11.5 million viewers overall) was one of the exceptions, likely edging up from last week’s delivery in the nationals and outperforming its score on the same night a year ago (when it did a 4.1 demo rating opposite ”American Idol”). ”Modern Family” was just 1 share behind ”Idol” for the 9 o’clock half-hour lead in adults 18-49, and pulled ahead of the music competition to rank as Wednesday’s No. 1 show in adults 18-34 (4.2/13). Elsewhere at the net, ”The Middle” (2.3/7 in 18-49, 7.5 million viewers overall) and ”Suburgatory” (2.3/6 in 18-49, 6.8 million viewers overall) both were down a tick week to week to hit season lows, but the combo outperformed last year’s tandem of ”The Middle” and ”Better With You.” And at 9:30, ”Happy Endings” (2.5/6 in 18-49, 5.7 million viewers overall) was in line with last week and up a bit from last year’s ”Mister Sunshine.” Subbing for ”Revenge” at 10 p.m., a ”20/20” special ”Before They Were Famous” did a 1.4/4 in 18-49, 4.8 million viewers overall , third among the major networks.

CBS finished second, getting consistently solid deliveries from ”Survivor” (2.8/8 in 18-49, 10.3 million viewers overall), ”Criminal Minds” (3.1/8 in 18-49, 12.6 million viewers overall) and ”CSI” (2.8/8 in 18-49, 11.8 million viewers overall). ”Survivor” was down a bit from last week’s season premiere, while the crime dramas matched or slightly exceeded their performances of last week.

At NBC, ”Whitney” (1.5/4 in 18-49, 4.1 million viewers overall) and ”Are You There, Chelsea?” (1.3/3 in 18-49, 3.4 million viewers overall) were down week to week. And that theme continued the rest of the way, as newsmag ”Rock Center” (0.6/2 in 18-49, 2.7 million viewers overall) and ”Law and Order: SVU” (1.6/4 in 18-49, 5.9 million viewers overall) hit lows. On the same night a year ago, ”SVU” did a 2.5 demo rating in the 10 o’clock hour.

CW saw no bump for the return of Chad Michael Murray to ”One Tree Hill” (0.7/2 in 18-49, 1.4 million viewers overall), and the finale of ”Remodeled” produced a slight uptick (0.3/1 in 18-49, 0.8 million viewers overall).

Preliminary 18-49 averages for the night: Fox, 5.1/14; CBS, 2.9/8; ABC, 2.4/6; Univision, 1.4/4; NBC, 1.2/3; CW, 0.5/1.

In total viewers: Fox, 15.9 million; CBS, 11.6 million; ABC, 6.8 million; NBC, 4.1 million; Univision, 3.5 million; CW, 1.1 million.