Monday, 21 December 2009

Caspian Gull (Larus cachinnans), 4cy, 21.12.2009, Aarhus Harbor

A beautiful and rather typical Caspian Gull found by the local birder Stephan Skaarup Lund (SSL). Most likely a 4th winter individual due to black marking in primary coverts, small and isolated mirrors on p10 and p9, relative large dark spot on upper mandible, and reminisces of classic winter neck-boa this late in season, where adults are typically in complete summer plumage by now.

One could state that the primary pattern is not spot on for cachinnans, but certainly within variation. There is black on p10-p5, the latter with a complete black subterminal band across the feather tip but being rather narrow on inner web. The black is extensive on the outer (leading) web, reaching the primary coverts on p10 and almost reaching on p9, and covering about half the exposed length of p8 and p7. The inner web has a reduced amount of black with the basal section fading from grey to whitish grey with a sharp division from the black, but the grey tongue are not that long on inner web of p10.

Adult Caspian Gull normally has large white mirrors on p10 and p9, although the p10 mirror usually merges with the white feather tip to give an extensive white tip; if not merged, the black between the white of tip and mirror is a very narrow bar like in this individual. The p9 mirror is better separated from the white tip but is sometimes split by a narrow black bar along the primary shaft. The small isolated mirrors on p10 and p9 could be related to sub-adultness in this gull.

1 comment:

Kent Olsen said...

Comments on ID and age at birdforum