By Chris Gibbins
Over the years there has been considerable interest in potential differences between various populations of Yellow-legged Gull Larus michahellis. Most of this interest has focussed on the western populations, notably differences between those on the Atlantic Islands, the Atlantic coast of North Africa and Iberia and those in the western part of the Mediterranean basin. This interest and attention has resulted in some populations being generally regarded as worthy of species status (e.g. Azorean Gull). However, what happens to the Yellow-legged Gulls as you go further east is a subject that has attracted much less attention.
Plate 1. Adult Yellow-legged Gull, Istanbul. Might there be differences between birds here in the eastern part of the species’ range and those we see in the west?
Differences in the primary pattern between eastern and western Yellow-legged Gull populations have been acknowledged for some time. Based on material in BWP and Lars Jonsson’s 1998 paper, Malling Olsen and Larsson (2003) conclude that:
‘Some east Mediterranean/Black Sea Yellow–legged show more white on wingtip, approaching Herring nominate argentatus and Caspian nominate cachinnans.’
If this additional white is marked and consistent, then clearly these eastern birds could create identification problems. The statement by Malling Olsen and Larsson (2003) that particularly caught my eye when I first read it was this one:
‘In the Balkans and the Bosphorus (the Yellow-legged Gulls are) said to show intermediacy with Caspian, but no details given (Cramp and Simmons, 1983) ’.
So, what exactly is being implied here? Are they really intermediate? How Caspian-like are they? Could there be extensive hybridisation going on in this area? I decided to go and look… This post shows a few pictures and presents some observations of the birds I encountered during a short trip to Istanbul in early 2014.
The Yellow-legged Gulls in Istanbul showed less black and more white in the wingtip than western birds. A deep black band across Primary 5 (P5) is one of the defining features of Yellow-legged Gull (e.g. Plate 2) and something we use in western Europe to distinguish this species from Herring Gull. However, my estimate (from a sample of 50 adults) is that 30% of birds in the Bosphorus lack this feature, instead having either a narrow and broken band or black only on one web. According to Jonsson (1998) such a pattern is seen ‘only rarely’ in michahellis. Another notable feature of the birds in Istanbul was the preponderance of a long pale (silvery grey) tongue on the inner web of P10. In western birds this tongue is typically short and therefore not eye-catching (Plates 2 and 3): in most it is somewhere between 25 and 50% the length of the exposed feather and tapers evenly to a point. However, many of the birds in Istanbul showed a striking long tongue, most frequently between 50 and 75% of the length of the feather. In some it was pointed but in some was evenly broad along its length and curved/rounded at the end (Plate 7) – so very like the tongue seen on Caspian Gull.
Many birds combined this tongue with a long, wholly white tip to P10 – again just like Caspian Gull. Such a wholly white tip is infrequent in western birds, in which the common pattern is a white mirror isolated from the feather tip by a black bar.
Many western birds lack a mirror on P9 (up to 50% according to Malling Olson and Larsson (2003) or have just a small mirror, often confined to one web. My estimate is that around 90% of the birds in Istanbul (from a sample of 50 adults photographed) have a mirror on P9 and typically this is large (extending across both webs); again this adds to the ‘more white, less black’ appearance of the wingtip.
Plate 2. Typical western Yellow-Legged Gull (N.E. Spain). Note the deep black band across Primary 5 (P5), the black bar across the tip of P10 and the small mirror on P9. The P10 tongue is narrow and pointed.
Plate 3. Another fairly typical western Yellow-Legged Gull (N.E. Spain); note the short grey tongue on the underside of P10. This short tongue and the black bar across the tip mean that black dominates the feather.
Plate 5. Yellow-legged Gull, Istanbul. Note that on this bird black on P5 is confined to the inner web. Combined with the long unmarked white tip to P10 and the large mirror on P9, this creates the general impression of relatively little black and lots of white in the wingtip.
Plate 7. Yellow-legged Gull. Istanbul. Note the long and broad silvery tongue on the underside of P10, combined with a long unmarked white tip to this feather. Together these features make this feather appear just like that seen on Caspian Gull.
These preliminary observations (a sample of only 50 adults) suggest that some overall plumage differences may exist between western and eastern populations of Yellow-legged Gull (I’ve also noticed that many birds on the Black Sea coast of Romania have more white/less black in their wing-tip). Nonetheless, it is also evident that there is considerable variation within populations, with some birds in Istanbul looking just like those in the western Mediterranean (Plate 8). Moreover, very occasionally western birds can have rather a lot of white in the wingtip (e.g. Figure 27 in Jonsson 1998). So, the bottom line is that we should not get carried away: there may be ‘population averaged’ differences but much more work is needed to properly quantify these.
Could these birds in the Bosphorus represent some integradation with Caspian? I was careful to look at the structure of these birds and, to my eye, there was nothing Caspian like about them. Although there was of course individual variation, their bills were typically robust (e.g. Plate 9). I also listened attentively to their calls and was careful to observe the long call posture (both of which differ between Caspian and Yellow-legged). All of the birds I was able to study sounded and behaved just like the Yellow-legged Gulls we see in the western Mediterranean: none gave calls that sounded like Caspian Gull or produced weird intermediate or unexpected calls, and all kept their wings closed when long calling. Thus, despite their wing tip pattern, I came away happy that they were Yellow-legged Gulls.
What does all of this mean? Well, there are two very simple conclusions. This first is that a comprehensive study, to determine whether the differences between eastern and western birds are formally (i.e. statistically) significant, would be extremely worthwhile. The second is that the reported ‘intermediacy’ of birds in and around the Bosphorus appears limited to the primary pattern; I saw no real evidence that these birds were hybrids with Caspian.
Jonsson, L. (1998). Yellow-legged Gulls and yellow-legged Herring Gulls in the Baltic. Alula 3, 75-100
Malling Olsen, K., & Larsson, H. (2003). Gulls of Europe, Asia and North America. Poyser, London.