There is an article in Birding World April 2010 about the ID and taxonomy of Bean Geese (thanks to Mike Leven for leaving this in our office!) which could be useful in determining ID of these birds. Much of the following is taken from that article.
There are four or five recognised taxa: serrirostris and rossicus (Tundra Bean Geese), and fabalis, johanseni and middendorffii (Taiga Bean Geese). Johanseni is often considered a hybrid of fabalis and middendorffii, while middendorffii is sometimes proposed as another split (Middendorff's Bean Goose). The most likely taxa in HK are serrirostris and middendorffii, which breed and winter in East Asia. All features used for ID could vary slightly due to age, sex and posture, so need to be used carefully. Of the Mai Po birds, I think one is a first-year and three are adults.
These two taxa differ significantly in size. Male serrirostris is quoted in Brazil as weighing 3.2kg, female middendorffii is almost 50% heavier at 4.6kg. As well as looking large, middendorffii should also look long-necked, whereas serrirostris should look short-necked.
Bill structure should also be useful for separation. Middendorffii should be very long and heavy billed, with a straight or slightly convex culmen; the bill should be approximately 50% of the total head and bill length. The cutting edge of the lower mandible should be strongly curved and the lower mandible should be slightly convex, bulging below the bill. Serrirostris should have a shorter, more conical bill, only 33-37% of the head and bill length. The cutting edge should also be curved. Both should have only a small amount of orange on the bill.
Plumage should be fairly similar between taxa. Middendorffii may show a ginger wash and possibly a more contrasting underwing.
So, applying this to the Mai Po birds. Although there does seem to be a slight difference in size, I am not sure that this is as much as expected for the difference between middendorffii and serrirostris. More troubling, the bird with the most serrirostris- like structure (i.e. short, thick bill and neck) does not appear to be the smallest. None of the birds show the short, conical bill I would expect of serrirostris. The rear bird on Martin's photo looks closest, but I think this is foreshortened slightly (and is still not as little as 37% of the head/bill). Koel's first photo shows the birds to all have a similar bill length. The 2 birds with the apparently very long bills may possibly the least like middendorffii in bill structure, with the bills not being particularly deep.
Overall, despite the apparent difference in structure and plumage, I am not sure whether the size and bill structure fit for the different bird being serrirostris. Perhaps this represents a different extreme of middendorffii, or perhaps there are elements of 'johanseni' in this bird. The other three birds may be more similar because they are related (parents with an offspring perhaps?). I am not a goose expert, though, so I would be interested to know what others think.
