brendank 22/04/2012 22:09
Common Tern (tibetana)
21 April 2012 - Southern Waters
I believe these are likely [i]tibetana[/i] although Robson shows the bill as rather strongly bicolored (red base and black tip) for [i]tibetana[/i]. Both Robson and Brazil state [i]longipennis[/i] has reddish-brown legs so I am not sure leg color is enough for subspecific identification on these birds.
[attach]11358[/attach]
[attach]11359[/attach]
[attach]11360[/attach]
AjaA 23/04/2012 11:38
Why do you think they are tibetana, excluding northern breaders?
We believe that the geograhical variation of Common Tern across Asia is reasonably correctly presented in the BWP update. So, some of the breeding birds as far west as the Ob basin have a blackish bill, as shown here:
[url]http://www.elisanet.fi/antero.lindholm/Linnut/Tiirat/hir_aasia.html[/url]
for example birds 9, 11 and 12.
The variation is largely clinal, and minussensis is a name inside this cline. That means that giving precise subspecific names in the wintering and migration areas is difficult. Tibetana may lie outside this cline, it should have a distinctly darker mantle than longipennis, but mostly red bill in summer, so it should be close to the nominate in this respect.
brendank 23/04/2012 21:30
There was this photo of the two presumed subspecies seen before. In this picture the difference looks rather marked.
[url]http://www.hkbws.org.hk/BBS/redirect.php?tid=5126&goto=lastpost[/url]
However, now that I see your photos of the terns from Russia it does seem that maybe the terns seen Saturday could be appropriate for [i]longipennis[/i]. Perhaps the bird with dark legs seen before was an aberrant i]longipennis[/i].