In the longer-than-Afghanistan war of the nightly variety-talk shows, The Tonight Show with Jay Leno remains ahead of Late Show with David Letterman , but both hosts have seen their ratings decline sharply in the past year -- Leno's more than Letterman's. In an interview with today's (Thursday) New York Times , David Poltrack, who tracks ratings for CBS, blamed the decline on the digital video recorder. "The DVR is the biggest competitor in the time period," Poltrack told the newspaper. The device is used at that hour mostly by the advertiser-courted 18-49-year-old audience. At the same time, an unnamed late-night executive told the Times , that group of viewers is not taping the late-night shows. "And you also have a firm sense that if something great does happen on one of these shows, you'll see it anyway online the next day."