Western Lesser Black-backed Gull (Larus fuscus graellsii), ad, 23.1.2015, Aarhus Harbor

This putative Western Lesser Black-backed Gull (Larus fuscus graellsii) was seen on January 23, 2015 on Aarhus Harbour in Denmark. Judged only on upper part colors we cannot safely dismiss an intergrade between intermedius and graellsii, but it is much paler than a classical Danish breeding intermedius.

Another Herring Gull (Larus argentatus) with yellowish legs

This European Herring Gull (Larus argentatus) was seen on January 3, 2015 at Havmølle Å on Djursland in Denmark. Note the yellowish legs, rather distinct primary pattern with much black, incomplete band on P5 and upperparts a little darken than on nearby argentatus (see video for color on upperparts).

Caspian Gull (Larus cachinnans), 2cy, 23.1.2015, Aarhus East Harbour

Herring Gull (Larus argentatus) with much white in primaries

This Herring Gull was seen in Hanstholm Harbour on 30 December 2013. It has a primary pattern which reminds of what is seen in some birds labelled as hybrid hyperboreus x argentatus (Viking Gull), but as it lacks a clear influence from Glaucous Gull other than in the outer primaries, it is considered to be European Herring Gulls with reduced or aberrant pigmentation.

Note the distinct thayeri pattern on P9 on the adult-type Herring Gull in the front 

Caspian Gull (Larus cachinnans), 2cy, 11.1.2015, Grenå Harbour

It is a very advanced bird, which has replaced nearly all visible coverts and even a tail feather. Note the bold, dark scapular centres and note that R4 in the left side of the tail has also been renewed.

Presumed hybrid Great Black-backed Gull (Larus marinus) x European Herring Gull (Larus argentatus) at Vejle Harbour

This presumed hybrid Great Black-backed Gull (Larus marinus) x European Herring Gull (Larus argentatus) was seen on 20 March 2011 at Vejle Harbour in Denmark.

Considering the massive amount of variation shown by some gull species, mixing genes can create quite a mix of plumage features. However, sometimes they can be surprisingly easy to figure out. This is due to the fact that a significant percentage of hybrids show obvious intermediate characters compared to the parent species.

While most birds of the Great Black-backed Gull x European Herring Gull combination give the impression of being a Great Black-backed-like gull with an intermediate mantle, it is also possible to find birds that are structurally more similar to Herring Gull (as shown below). The darker mantle of this hybrid alerts the observer to something interesting. However it is clearly much too pale to be a Great Black-backed Gull. The size (and relatively large bill) combined with pink legs, squarish head and primary pattern; seem to exclude a Lesser Black-backed parentage, leaving little doubt to this identity.

Other possible GBBG x EUHG hybrid candidates can be found here, here, here, here, and here.

Presumed hybrid Lesser Black-backed Gull (Larus fuscus) x European Herring Gull (Larus argentatus) on Mandø

This presumed hybrid Lesser Black-backed Gull (Larus fuscus) x European Herring Gull (Larus argentatus) was photograped on Mandø in the Danish Wadden Sea on 27 October 2011. It attracted my attention because of its dark-grey upperparts which were much darker than on the nearby European Herring Gulls but at the same time the bird was not a Lesser Black-backed Gull.

The possibility of it being a hybrid Lesser Black-backed Gull x European Herring Gull, seems to be the best fit because it was much darker grey compared to local Herring Gulls and it showed distinct pink-tinged legs, rather dark underside to primaries and a broad white trailing edge to secondaries as in Lesser Black-backed Gull and finally a Herring Gull JIZZ. In other words, it showed mixed characters of both species.

Herring Gull with a grey 'pseudo-mirror' on the underside of P9

A grey mirror found on the underside of P9 or P10 has been suggested as a supporting micro feature for distinguishing adult American Herring Gulls from European Herring Gulls. It is described as a somewhat isolated grey spot or grey hole inside the black pattern on the underside of P9 or P10. The grey spot is either completely surrounded by black or seen as a grey cut which is open towards the base of the feather. It may be present in both wings at the same time or only in one of the wings.

The feature is apparently rare in European Herring Gull, but has been seen in a few hybrids (or backcrosses) of Glaucous Gull x Herring Gull in Iceland. During Christmas holiday this year I looked through some more of my photos of apparent argentatus Herring Gulls, and came across the individual below which was seen on December 28, 2008 in Østerby Harbour on the island Læsø in Denmark. It represents the second European Herring Gull that I have ever documented with such a small grey ‘pseudo-mirror’. The first example was this one from the other end of the same island.

Another interesting character in this individual is that there is no mirror on P9. However, it is widely known that there is extensive variation in the primary pattern of European Herring Gull. We also know that precise pattern changes over years as documented by Maarten van Kleinwee