Bean geese (6) - an interesting pair on Shetland

Again some very interesting pictures of Shetland. Roger Riddington commented on these by e-mail:

On 1st February, a message went out on our grapevine about two Taiga Bean Geese, and that evening some pictures appeared on one of the Shetland birding blogs (http://www.shetlandbirding.blogspot.com/). See entry for 1st Feb (best to ignore some of the comments in the text).

I was intrigued by the pictures on the web since they seemed to show a rather skinny-billed bird that looked similar to the adult female Taiga in the group we corresponded about in December – whereas the other bird looked to have a bill structure more like Tundra. Anyway, a couple of days later I managed to get out of the office for a couple of hours and went to see them. In real life I was struck by two things – 1) that overall, in terms of overall size, body shape and structure etc, the two birds were pretty similar. I could see why people thought they were both the same form (and some people are of the view that they are both Tundras!). But through a telescope, the bill structure looked quite different, to me. The orange-billed bird has a much more slender bill, with a more or less entirely straight cutting edge to the upper mandible, a very slim, flat lower mandible, and I would say a bill depth at the base which is slightly but distinctly smaller than the second bird. The stripy-billed bird has a bill that is about the same length, but is deeper at the base, with a much deeper base to the lower mandible, and a very distinct curvature to the cutting edge of the upper mandible.

Based purely on bill structure, I felt that the ‘best fit’ solution was a petite female Taiga and a big male Tundra. But I can see that when you have two vagrant birds like this it is more often going to be the case that they are the same form. The similarities in overall shape and size was also confusing. I would hesitate to say that the two were ‘paired’ but they were certainly together the whole time I watched them – if anything, the stripy-billed one was slightly larger/heavier and perhaps spent a little more time alert – reinforcing my tentative view that it might be a male.

The light wasn’t great, so my pictures are again a bit grainy and distant – but if you felt able to comment I would be interested in your opinion.

All the best,

Roger

This is one of the bills you wouldn't expect on such a body! 

Hi Roger,

Thanks again for sending me these pictures. Your description is quite complete. The only thing I would say: yes, this is a pair of fabalis and the male has a type D bill. 10% of fabalis has this colouration.
It would be strange to seperate the birds into two species, since their is no known record of an hybrid pair of fabalis and rossicus (Heinicke in litt) and they behaved as a pair.

Except the bill, everything on this bird points to a classical, rather big Taiga Bean. Still, the variability of the billstructure in both rossicus and fabalis isn't described and depicted properly in the literature.
Thick lower mandibles like this male has, are very rare in fabalis. Pro fabalis is the concave culmen and the overlapping tip of the upper mandible. But such an deep and bowed base of the lower is
surprising. Note that the orange bills always look a bit more slender as the result of a kind of visual effect.

These pictures do stress my vision to have a databank once, with hundreds of heads 'en profile' of both species to depict their variability. For example, have a look at those pure fabalis flocks: http://www.birdphotos.dk/index.php?view=category&catid=910&page=3&catpage=1&option=com_joomgallery&Itemid=3&lang=en
Thanks, best regards,

Albert