Regarding Tony's point about the flank-pattern, the rather plain flanks on one of the Tundra Bean Geese may indicate that it's a juvenile.
I think the difference between these two very similar "species" is well shown in the last of John Holmes's photo above. My understanding is that the bills are the crucial feature. If you look at the photo, the bill of the Tundra Bean Goose on the right has a different shape to the Taiga Bean Goose on the left. There is also a difference in the amount of orange on the bill and how far the orange extends towards the nostrils. Also, the line formed by the closed mandibles is pretty straight on the Taiga Bean Goose but is somewhat downturned on the Tundra Bean Goose, giving a characteristic grin-like expression. The latter feature is clearly visible in other photos and may well be diagnostic.
david
[ Last edited by ddavid at 4/11/2010 17:58 ]