Actually, closer to 48 million "birders"
The U.S. Fish and Wildlife Service reports that 71 million Americans participated in wildlife watching activities in 2006. ... Birding undoubtedly accounts for a large segment of all wildlife watching.Actually, the service's survey pinpoints on page 36 the number of survey respondents who said they observe birds at home and on trips: 47.7 million. The survey results also said, "A large majority, 88 percent (41.8 million), observed wild birds around the home, while 42 percent, 19.9 million, took trips away from home to observe wild birds."
You can find lots of information in the PDF of survey results, released in October 2007. I, however, remain skeptical about the 48 million birdwatchers. That figure in no way reflects the number of birding magazine subscribers, festival attendees or tour participants.