It has been stated many times before, but I’ll do it again. When evaluating jizz one should always remember that single situation photos will not always do a bird justice. It is always best to make a judgement based on multiple photos capturing different posture/behaviour/situations if at all possible.
Several have stated that head profile in the Danish gull is very herring gull-like and head size is relatively large. Even though this is certainly not uncommon in male Thayer’s Gull it has provoked doubt and fuelled hybrid speculations among birders who live in hybrid hotspots such as the US West Coast where hybrids plumage-wise almost make a complete match with the wintering Thayer’s and where people are left with size, proportions and jizz to make a safe ID.
I have trouble understanding that even though so many experienced gull watchers have joined this discussion the main argue against the Danish gull being a Thayer’s is size and proportions of especially head and bill. I expect we all have experience with gulls changing appearance in relation to behaviour and posture.
For comparison I have posted two photos of an Iceland Gull feeding in the same place in the container just few minutes before. It was normal round headed when walking in safe distance; however, when it joined the feeding frenzy is suddenly transformed into a large-headed bird with flat forehead. It was striking how the appearance and jizz changed, especially the impression that the head suddenly appeared much larger relative to the body when the bird feed.
I’m not stating the Danish gull transformed into a small petit female, but the relative large headed appearance certainly changed. When watching the gull on the roof I actually had to view my camera display for finer details to make sure it was the same gull. I acknowledge that proportions and size strongly indicates that it is a male, but I believe that the scary appearance is also caused by an artefact created by posture and behaviour and possibly also the telephoto effect induced by my Canon 500 mm lens.
I might be wrong, and then please refresh my memory, but in my humble opinion there are no plumage characters which actually directly speak against Thayer’s. All plumage characters that people have found not supportive of Thayer’s have been matched in other Thayer's Gulls with safe ID.
People have e.g. argued that the outer tail feathers and the details in the venetian blind in the primaries is not pro Thayer’s. Therefore I have also collected wing and tail shot. I don’t see any difference in these characters compared with several examples of Thayer’s photographed away from the hybrid hotspots.
I completely acknowledge that people living in a hybrid hotspot use a very restricted view on what a true Thayer’s look like and due to scary hybrids feel that they are mostly left with size, proportions and jizz to make a safe ID. However, 'the swarm of hybrids on the American west coast is mostly a local problem, not a worldwide one' as stated by Peter Adriaens.