The peacock's feathers are fraying. After more than a decade as the dominant broadcast network, General Electric Co.'s NBC is having a rocky fall. So far this season, the network has endured double-digit ratings declines in both total viewers and viewers aged 18 to 49, the demographic that advertisers pay a premium to reach. Particularly troubling: For the first time since 1987, NBC is losing the ratings race on Thursdays, the night that by far draws the most advertising dollars and has been the bulwark of NBC's "Must See TV" franchise. NBC Universal Chairman Bob Wright, publicly putting a brave...