Friday 26 February 2016

Tundra Swan: hybrid columbianus x bewickii pair?

Last weekend (Feb 21) I came across what I was certain must be a Whistling Swan Cygnus columbianus columbianus in a flock of about 80 Bewick's Swans C c bewickii; a fairly rare find in Japan. Interestingly it gave the impression of being paired with a dark-billed type (a "darky"; after Scott, 1966) Bewick's Swan C c bewickii. I have to say at the outset that they weren't part of a family group (there were surprisingly few first winters in a flock of about 80 swans), so the only indication that they might have been a pair is that they seemed to have a close affinity. They grazed together, which may have been coincidental, and on one occasion walked a about 20m through the flock as if a pair - so nothing conclusive, just an unsubstantiated hunch.

The second bird had very limited yellow in the bill by bewickii standards, far less than any other bewickii in the flock or that I'm used to seeing, however according to Evans and Sladen (1980) it is within the range of extremely 'dark' bewickii but outside that of an extremely yellow columbianus.

The two swans in question, columbianus on the left, the 'darky' bewickii on the right and a 'yellowneb' between them.

I can't help wondering whether it's possible for some western columbianus to show this extent of yellow or if extreme 'darky' bewickii might select the darkest possible mate? Mikami (1989) mentions three of four mixed pairings he recorded, three of the bewickii were 'darky'. An alternative might be the bird I saw is a hybrid which "recognizes itself" as columbianus, so its choice of mate would be no surprise.

The closest I came to getting both in profile. The 'pennyface' in the centre was the only one I noticed in the flock though I wasn't searching too hard in the rain.

The most convincing Whistling Swan I've seen was Shimane seven years ago. Not only was the bill pattern perfect but it was clearly larger than the accompanying Bewick's despite there being so much overlap in size between the two races.

No doubt about this one; my only "classic". This not only had the perfect teardrop but was distinctly larger than the bewickii. The bill seems deeper-based, as I've read it can be, but it's hard to know for sure how much of this might be an illusion caused by the different colouration. I've also read the head shape can look slightly different, something I'm always wary of, and all the birds I've seen do seem to have more of a forehead, a slight coiffure whereas the bewickii have a straighter profile.

My only other columbianus were eight years ago when I had a pair with a single young bird, again at Lake Biwa, and again one looked good while the other had a lot of yellow on the bill. However the yellower-billed bird does seem to be within the range of columbianus.

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