brendank

Great photos everyone!  Hope these birds stick around until this weekend.

I am having a hard time being convinced that there are two species here.  I think to say that there was mixed species flock here there would have to clear structural and plumage differences between birds.  To be honest, I don't see any plumage differences between these birds.  I would admit that two of the birds appear slightly larger and thicker necked but I think in most populations of geese there is some variability in structure and size and I suspect the difference seen in these birds is well within the with-in species range.

brendank

Quote:

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.

I agree with John completely on this point.  I think his response is very well stated indeed.  I don't know that much about Eurasian Geese species.  But I do find that in North American Geese species such as frontalis Greater White-fronted Geese and atlanticus Snow Goose that there can be quite striking variation in size, bill length, and neck length within species.  

Furthermore, it is my understanding that in Europe Taiga and Tundra Bean Geese rarely mix even in wintering flocks so I find it unlikely that geese from two different distinct subspecies would mix and then become vagrants in HK.