This might help some. The two Khaki females have brownish bills while the Khaki drake has a green/grey bill. The other two are Swedish ducks. The drake again has a green/grey bill and the female (crested) has the darker grey bill. This seems to be consistent in all my breeds. My Cayugas have green/grey bills on the drakes and black bills on the females.i have 2 that have olive colored bills and the rest have grey bills