48 million birdwatchers put $36 billion into the 2006 economy
"Birding in the United States: A Demographic and Economic Analysis," a 20-page addendum to the 2006 national survey of fishing, hunting and wildlife-watching recreation, clarifies that the national survey considers a birder to be an individual that has "taken a trip 1 mile or more from home for the primary purpose of observing birds and/or closely observed or tried to identify birds around the home." That excludes, for instance, someone who notices a gull in a store's parking lot and calls it a "seagull."
What do you think of that definition? USFWS calls it a conservative definition. I wager that some birders consider it too broad.