This is crude, but let's assume that there are three groups of people that buy duck stamps
1: Waterfowl hunters
2: Stamp Collectors
3: Birders / other non-waterfowl hunting natue recreationists
In Louisiana for example there were an estimated 65,000 +/- 9% duck hunters:
http://www.wlf.louisiana.gov/pdfs/hunting/programs/waterfowl/2005 USFWS Harvest Survey.pdf
and 96,115 duck stamp sales.
http://www.wlf.louisiana.gov/pdfs/hunting/programs/waterfowl/2005 USFWS Harvest Survey.pdf
so in Louisina anyway about 30,000 people bought duck stamps that did not hunt that year.
Birder's do buy duck stamps to gain entry to the National Wildlife Refuges, and are actively encouraged to buy them in forums such as BIRDCHAT or by the National Fish and Wildlife Foundation. I couldn't find any study that looked at the breakdown of duck stamp buyers by 'reason for purchasing'.
There has been talk for years about creating a bird conservation stamp, but it hasn't taken off, nor has a Pittman-Robertson type tax on birdseed, feeders, binocs, etc. Both would be a great idea to generate funds for land purchase. It's going SO fast these days. Development is insane almost everywhere in the US.