Ämmässuo, Espoo 17 December 2011

The bird photographed below was first briefly seen a week earlier. Clearly it has something to do with Caspian Gull, but some characters do not seem to be so good - especially the quite distinct gonydeal angle and uniformly barred axillaries.

A new and eagerly awaited article (Gibbins, Neubauer and Small in British Birds December 2011, Identification of Caspian Gull, part 2: phenotypic variability and the field characters of hybrids) discusses just these problems. I am trying to use their trait scoring system on this bird:

Scapular moult. There are one or two rear scapulars unmoulted, as is often the case with first year large gulls. I suppose this means score 1.

Greater covert pattern. This is difficult to decide, but score 1 is the best fit.

Ventral bulge. I have never thought much of this as a character for separating these two species, and - in any case - am unable to decide whether this individual has one or not. Let's guess 1 to on be safe side towards Herring Gull.

Primary projection. Measured from photo 0.62. Score 0.

Moult, greater coverts. Score 5.

Moult, median coverts. Score 5.

Moult, tertials. Score 3.

Darkness of head and body. Score 1  - I think quite clearly in this case (the head and neck are very wet, a week earlier the white head was more apparent).

First generation tertial pattern. Score 1.

Second-generation scapular pattern. Many of the feathers show a "strong pattern", but many are very plain. The area of plain feathers is large, because the feathers are large, but the amount of feathers showing "strong pattern" seems to be larger, so the score is 3.

Table 2 ends here, but two other characters are scored, explained in the Table 1 (which otherwise is for adults):

Bill length. Measured from the photos, 2.17, which means score 2 (intermediate length).

Leg length. There is not much guidance how to decide this (which is also not so easy to use in the field either). The legs do not seem to be very short or very long, so I score them as 1.

This gives overall score of 24,  which is inside the cachinnans area (although towards the upper end), and outside the argentatus area.

Otherwise, I would say that the underwing of this bird is otherwise inside the normal cachinnans variation, but possibly not the scapulars. The upperwing pattern (inner primaries) seems to be quite good for Caspian Gull and also the tail (broadish black terminal band and smallish vermiculation inside it). The final useful character of Caspian Gull - the call - was not heard from this individual.

The bird appeared again 21 January 2012. The following five photos were taken then.