[EBB Sightings] question about winter goldfinch ID

I've got a question about identification of winter Lesser and  
American Goldfinches. My Sibley field guide and the San Diego Natural  
History Museum (http://sdnhm.org/research/birdatlas/focus/ 
goldfinches.html) say that the American Goldfinch has a pale yellow  
bill in winter, and that the Lesser Goldfinch has a dark grey bill in  
all seasons, and that's what I've used to tell them apart. BUT the  
Cornell web site says that the American Goldfinch has a dark grey  
bill in winter, and most of the photos on the web of winter Americans  
show them with dark grey bills.

Who has the authoritative answer? And how do folks ID winter American  
vs. Lesser goldfinches?

I've got a huge flock of goldfinches in my yard right now, and I'd  
love to know which species predominates.

Charlotte Allen