Why I really went to Finland

Baltic Yellow-legged Herring Gulls and the 'omissus' question

OK, I may not have come back with any cracking photos of owls but admit it, this is what you've all been waiting for - my yellow-legged 'Herring Gulls' from the Baltic.

Even occasional visitors to this blog will have noticed a regular feature - the mystery 'yellow-legged' gull challenge. This is when I post a photo of an adult gull with a medium grey mantle and yellow legs and proceed to convince myself at least that it's not Larus michahellis but a hybrid, probably argenteus x graellsii. So as I'm quite accustomed to seeing non-michahellis gulls with yellow legs, I felt comfortably at home in Helsinki recently when faced with a whole freakish series of yellow- (or yellowish-) legged argentatus types and their alarming progeny. No surprise really, the Finnish gulls with their yellow legs have been the subject of much head-scratching for many years; they even have name - 'omissus' -  which appears to have been originally applied to a different population and, since no proper description was published, is invalid anyway.

The taxonomic status and origin of these birds has been discussed since at least the early 20th Century and is seemingly no nearer resolution; if you really want to give your brain a workout, I recommend Jonsson 1998, also Panov & Monzikov 1998 for a Russian perspective, and Voipio 1993 for a historical review, the name 'omissus' first appears in Pleske 1928. Be warned, though, it's all a bit of a car crash. [Edit: should have mentioned, but forgot, Mierauskas & Greimas 1992].

Meanwhile, it's time for the gulls themselves; I spent a few hours over two days by the quay in Helsinki getting to know a small group of about 30, mostly 2nd cal.yr./1st cycle birds with about 10 adults and just one 3rd cal.yr./2nd cycle (which never came close enough to photograph). I'll start with adults.

Larus sp. adult - 14/3/2015 Helsinki, Finland

Larus sp. adult - 14/3/2015 Helsinki, Finland

Larus sp. adult - 17/3/2015 Helsinki, Finland

Larus sp. adult - 14/3/2015 Helsinki, Finland

Larus sp. adults - 17/3/2015 Helsinki, Finland

Admittedly it's a small sample but a few things stood out:

None had the obviously pink legs I would associate with either British argenteus or Scandinavian argentatus Herring Gulls. The leg colour ranged from a pale, washed out pinkish-yellow to a bright michahellis yellow. Mantle shade was consistent and darker than British argenteus, typical of nominate argentatus in fact, the primary pattern was also typical of argentatus - all birds had much white on P10, either a large white tip or a large mirror with just a small black band before the feather tip, there was little or no black on P5, the orbital ring and gape appeared to be consistently red.

Now a selection of 1st winter birds and these were, if anything, even odder.

Larus sp. 1st cycle/2nd cal yr - 14/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 14/3/2015 Helsinki, Finland (same bird as above)

Larus sp. 1st cycle/2nd cal yr - 14/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 14/3/2015 Helsinki, Finland (same bird as above)

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland (same bird as above)

Larus sp. 1st cycle/2nd cal yr - 14/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 14/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yrs - 17/3/2015 Helsinki, Finland

Larus sp. 1st cycle/2nd cal yr - 17/3/2015 Helsinki, Finland

There's some variation in the base colour of the head and underparts as well as the degree of wing covert barring, not unlike what you'd expect in a Herring Gull at this time of year. Many have fairly dark coverts with thin simple fringes not unlike michahellis or cachinnans. The commonest scapular pattern seemed to be a thin sub-terminal anchor mark, often against a brownish background although many were pale. I could no sign of any moult in the wing coverts and some had retained a few juvenile scapulars. The big surprise was when these birds flew, showing little sign of Herring Gull type pale inner primaries - they were more or less all dark, like a Lesser Black-backed or a Yellow-legged Gull. The tail consistently showed a blackish band beyond a heavily black-barred white base, likewise the upper tail coverts were well marked.

Given that these birds are often alleged to originate from hybridisation between argentatus and cachinnans, I've got to admit I could see little sign of the latter other than the bare part colour in the adults and the primaries of the 1W.

So what are they? Well, what about the argentatus x cachinnans hybridisation theory? It seems to be accepted that numbers of pink-legged coast-nesting argentatus increased in the Baltic during the 20th Century, while at the same time yellow-legged colony-nesting cachinnans have spread northwest from the Caspian area. However, it appears that yellow-legged gulls were already nesting on the bogs and lakes of the eastern Baltic region at least 200 years ago, whether these were actually cachinnans or something else entirely remains to be proven. Pleske's original 'omissus' actually refers to birds collected on the Arctic coast of the Kola Peninsula and islands in the White Sea. Confused? You should be. As I said earlier - car crash.

References

Jonsson, L. 1998: Yellow-legged gulls and yellow-legged Herring Gulls in the Baltic. Alula 3:1998

Mierauskas, P. & Greimas, E. 1992: Taxonomic status of yellow-legged Herring Gulls in eastern Baltic. Dutch Birding 14: 91-94

Panov, E.N. & Monzikov, D.G. 1999: Intergradation between the Herring Gull Larus argentatus and the Southern Herring Gull Larus cachinnans in European Russia. Russian Journal of Zoology 3:129-141

Pleske, T. 1928: Birds of the Eurasian tundra. Mem. Bost. Soc. Nat. Hist. 6: 1-485

Voipio, P. 1993: Differences in ecological properties in the Herring Gull (Larus argentatus) as a basis for explaining and predicting colonization events - A case history in retrospect. Ann. Zool. Fennici 30: 3-15