We saw a coot with a yellow bill very close up in the impoundment directly in 
front of parking, about 3 weeks ago; the coloring appeared natural. We 
wondered about that at the time. It closely accompanied by what appeared to 
be a female mallard and quite separated from the major coot group in the large 
impoundment. I also wondered about that. I checked Sibley at the time which 
shows no variations of that sort.

Phil Brody