Barrow's Golden EyeThe bill colour of female Barrow's is only an aid to the identification and it is not diagnostic. Most important is to look at the head shape. The head of the female Barrow's Goldeneye has an oval shape created by a steep forehead, flat crown and elongated hind neck feathers like a mane. The latter gives the appearance of a swept back crest, similar to the male Barrow's and female Hooded Merganser. The head shape of the Common Goldeneye lacks the "puffiness" of the female Barrow's. It has a rounded rather than flat crown and a sloping forehead.
For more information see identification articles below.
Di Labio, Bruce, Ron Pittaway & Peter Burke. (1997). Recognizable Forms: Bill Colour and Identification of Female Barrow's Goldeneye. Ontario Birds, Vol. 15, No. 2, Pages 81-85.
Martin, Paul R. & Bruce M. Dil Labio. (1994). Identification of Common x Barrow's Goldeneye Hybrids in the Field. Birding, April 1994, Pages 104-105.