erythropus: smallest WGO, pink bill, yellow
eye-ring
albifrons: smallest and
lightest GWGO, pink bill
frontalis: very similar to albifrons, but larger, darker,
longer bill, pink bill
elgasi/gambeli: larger, longer
bill, darker than frontalis, size and plumage like flavirostris, but pink bill
flavirostris: larger, longer
bill, darker head & upperparts than albifrons, orange
bill
Bill color is extremely dependent on light
conditions!
Madge S & Burn H
(1988): Wildfowl. - London.
Ogilvie M & Young S
(1998): Photographic Handbook of the Wildfowl of the World. - London.