From the Associated Press as featured in USA TODAY:
One in every five Americans watches birds, according to a new report from the U.S. Fish and Wildlife Service called “Birding in the United States: A Demographic and Economic Analysis.” The report claims that 48 million people birdwatch, remaining at a steady 20% of the U.S. population since 1996. By region, the report found that birdwatching was most popular in the South, with an estimated 33% of the population taking part in the activity. The report is an addendum to the 2006 National Survey of Fishing, Hunting, and Wildlife-Associated Recreation. The survey is the eleventh in a series of surveys conducted about every five years that began in 1955. The survey is conducted by the U.S. Fish and Wildlife Service in partnership with state wildlife agencies and national conservation organizations.
For the complete U.S. Fish and Wildlife Service report see http://library.fws.gov/Pubs/birding_natsurvey06.pdf