Drudge is reporting the quick slide of the Katie Couric-helmmed CBS Evening News into third place. According to overnight Nielsen ratings, the Couric broadcast finished in last place on Monday evening, trailing ABC’s World News with Charles Gibson and NBC Nightly News with Brian Williams, which returned to first place in the ratings.

While I’m no Couric fan, her fall into third place was not unexpected. Viewer curiousity prompted many to tune in for her debut last week. When they sampled the CBS broadcast, most simply reverted to habit, reflected in higher ratings for ABC and NBC in the days that followed. Not even the most optimistic CBS executive believed that Couric could grab the top spot and hold it from day one.

History shows that ratings shifts in the evening news race occur slowly, over a period of years. When Dan Rather inherited the CBS Evening News in 1981, he held a commanding advantage over his rivals at ABC and NBC. When Peter Jennings became solo anchor of World News Tonight in 1983, it took him about four years to overtake Rather. It took NBC’s Tom Brokaw even longer to overtake Jennings. Meanwhile–as virtually every media pundit has noticed–the overall audience for the network’s evening newscasts has plummeted. Brokaw’s audience in 2004 (when he retired as anchor of the first-place Nightly News) was smaller that 1984, when he was running a distant third. All told, audience levels for the evening news are about one-third lower than they were 20 years ago. Millions of Americans have simply given up on the broadcast networks as a primary information source–and never looked back.

We don’t expect Ms. Couric to buck that trend. The real question is whether she can be competitive in a three-horse race, or if her audience will sink back to Rather-esque levels. If her ratings fall that far, then CBS will become nervous. The network can’t afford another decade with an evening news program that’s buried in the Nielsen basement. Low ratings means millions of dollars in lost ad revenue. It also creates serious problems for CBS affiliates; poor ratings for the Evening News translates into smaller audiences for local news broadcasts or primtime access programs, both critical revenue sources for local stations.

Bottom line: Ms. Couric probably has about five years to turn around the Evening News. If she’s still in third place in the Fall of 2011, her days in the anchor chair will be numbered. At $15 million a year, CBS can afford the experiment for only so long. At that point, the network will probably announce her “promotion” to full-time correspondent on 60 Minutes, an immensely profitable broadcast that can more easily afford–and absord–over-priced talent. The Evening News (if it survives) will morph into something more affordable, at least in terms of its anchor. And that raises another question: exactly who is CBS grooming for the top job. One reason the network hired Ms. Couric is that Dan Rather jealously guarded the anchor chair and never let CBS develop an eventual successor, like NBC did with Brian Williams. It will be interesting to see who emerges as the top substitute for Ms. Couric. If she follows the Rather model, she’ll pick someone who’s competent, but not a threat to her position. That would leave CBS in the same boat when she eventually leaves the Evening News.

By MYLIFE