Gene:

Here is a report of use for U.S., which you can use and extrapolate based on 
MD data:    http://library.fws.gov/nat_survey2001_birding.pdf

It is from 2001, and is a 5-year FWS study summary.  I think the report 
mentioned by Gail may be the next one that follows this one up.

You can find more info at:  http://firstgovsearch.gov/search?
query=Bird+watching+report&v%3Aproject=firstgov&affiliate=fws.gov

Cheers-
Jeff Shenot
Croom MD