TWO bird identification problems connected to European species here in south east Michigan on the same day: A possible Mew Gull and a potential Greater White-fronted Goose! I didn't get to see these birds, but still there's plenty to write about...
OK, first things first and this post will contain a few remarks on the Common - Mew Gull problem and also a little bit of Common - Ring-billed Gull talk will be included. It is not intended as an identification article, I'd just like to point out a few things and difficulties that I stumbled across in the past few years birding along the Baltic coast of Germany, trying to find a Mew or Ring-billed Gull amongst the many Commons.
The following remarks about the Common - Ring-billed problem therefore are sort of the wrong way around for Michigan: I am NOT investigating to what extend an ordinary Ring-billed can show characters of Common but - maybe a bit less useful in a North American context - how often Ring-billed Gull characters can be shown by Common. I really hope this post will not be used to "string" a Common Gull here ("sure, it's not really typical, but Jochen said..."), but it might be of some help nevertheless to demonstrate that by far not all the Commons look the way they are depicted in Sibley...
The last remarks on Common / Mew are hoped however to be not completely useless here as I suspect the probability of a Common is not significantly lower than of a Mew.
The first step in identifying a Common (and Mew as well) here in Michigan is to exclude Ring-billed. The most important identification criteria are on the wing and the head (OK, that was dumb: where else on pretty much any gull?) and can be summarized as follows:
a) The eye is pale on Ring-billed and dark on Common and also Mew.
b) The bill is bright yellow with a broad black ring close to the tip in Ring-billed, whereas it is paler yellow on Common and Mew and shows a weaker, often incomplete black ring.
c) The bill is much stronger in Ring-billed with parallel edges at the base whereas the bill of Common and Mew Gulls seems to narrow rather evenly towards the tip.
As a matter of fact, all but one or two of the Common Gulls I studies in Germany in the last few years showed a rather bright yellow bill and a complete black ring. Even though this ring might not be as pronounced as on Ring-billed Gulls, it is still rather obvious and in my opinion close enough to Ring-billed in some cases to render this feature pretty much useless for separating the two. Here is an example of a Common Gull from Germany with a very pronounced black ring on the bill, this kind of yellow is very much the norm though.
Here is a very heavily cropped section of the head, showing the broad and complete black ring on a bright yellow bill.
The dark eye however and details of the wing clearly identified this bird as a Common Gull.
The following image (again followed by a cropped section of the bill and eye) shows the typical bill structure of Common Gulls nicely, with the upper mandible narrowing very evenly in a nice and smooth curve from the base to the tip. This bird also shows a dark eye typical of Common.
The bill structure might really be the single most definite feature of a Common. In other words: if your potential Common Gull has parallel upper and lower edges of the bill (reminiscent of Herring Gull), it very, very likely definitely will NOT be a Common (or Mew) Gull! Do not be discouraged if your potential Common shows a complete black ring on the bill because a whole lot of them regularly do.
The pale eye is also a good identification mark as the vast majority of Common Gulls show dark eyes. If you are very close to them (e.g. hand feeding them in winter) in bright sunlight that's shining directly into their eyes, they might appear to have somewhat paler (olive green) irides. But these are far from the bright yellow staring eye of the Ring-billed. There are however examples of Common Gulls with more bright paler eyes but these can be considered exceptional. If therefore your Gull shows a pale eye, chances are it is NOT a Common...
a) Ring-billed has a small white mirror on P10 (the outermost primary) and occasionally an even smaller one on P9, Mew and Common have much larger white mirrors, Common sometimes even one on P8.
b) Ring-billed has no white between the black tips and the gray bases of the outermost primaries, Common and Mew do to a varying extend.
c) The mantle in Common is darker than in Ring-billed.
d) The white trailing edge on the secondaries is broader on Common and Mew than on Ring-billed.
If a bird shows all of the characters mentioned for Common it is very likely a Common, but it should be stressed that I have found ALL of the Ring-billed characters mentioned above occasionally on Common Gulls along the Baltic coast, so each character in itself is far from diagnostic.
The following image is of a typical western Common Gull (Larus canus canus). It shows three white spots on the outermost primary tips, white between the black and gray on the central primaries and a rather broad white tip/trailing edge to the innermost primaries. If your Gull shows these features, I suppose you're doing well on your way to identify a Common Gull and all you have to do then is eliminate Mew (see below). If one of these features is missing, it might still be a Common, but of course it makes it much more difficult to get to a certain (and credible) identifiaction.
Many Common Gulls, presumably of a more eastern origin or even of the Asian subspecies heinii, show white spots on only the two outermost primaries and these white spots are also smaller. The white trailing edge on the primaries (not the secondaries) is more narrow and the white between the black wingtip and the gray base is more restricted (sometimes even missing!) and shaped like a narrow half moon. These are more likely to be mistaken for Ring-billed, but this is also a form with a very dark mantle, so that should be rather obvious amongst a flock of Ring-billed (but hard to assess on single birds in bright sunlight). The following image shows such a potentially eastern Common Gull. Note that the black area on P8 is far more extensive on this form than on classical western Larus canus canus.
One of these presumable eastern Commons once almost lured me into identifying it as a Ring-billed (see image below): There was only one white mirror on the outermost primary, no white between the black wingtips and the gray base and a very narrow white edge to the inner primaries. The white edge to the secondaries however was very broad and checking the gull carefully e.g. for characters on the head, it was apparent that this was an ordinary Common Gull whose outermost primary was missing (probably broken off) hence creating this misleading Ring-billed like pattern.
So, even if this is very unlikely, if your gull looks very good for a Common but shows only one white spot on the primaries, make sure it really has all its 10 primaries!
The Mew Gull problem
All of the above was jut a little bit of random thinking, but the following might be interesting regarding the Mew Gull from Port Huron:
Occasionally western Common Gulls can show a pattern superficially similar to Mew Gull.
First of all, here is a link to an identification article on Surfbirds.co.uk, where all the important features to look for are mentioned. The problem with these features is that they require a very careful analysis of plumage details that are very likely impossible to check on a flying bird in the distance. One of the most useful characters to generally (not definitely) separate Mew from Common therefore appears to be the impression that the white trailing edge of the inner primaries crosses the black area of the central primaries to connect with the white spots on the outer primaries, producing a "string of pearls" effect. In Common Gull, the white "tongues" on the central primaries are usually less pronounced and do not connect with the white spots on the outermost primaries, so the latter will always appear isolated and completely surrounded by the black wingtip.
This general approach to separate a Mew from a Common might be true for the very vast majority of Common Gulls but once really got my pulse going for a few days on the Baltic coast. It was not until I got a few digital shots of the birds (allowing for a detailed analysis of its primary pattern) that I was able to definitely identify it as a Common with an unusually large amount of white on its wing. The more useful images of this strange bird are shown below.
Now, what was so unusual and lured me into believing it might be a Mew?
The white mirrors on P10 and P9 were very extensive and even P8 showed an unusually large white spot. The white "tongue" between the black tip and gray base of P7 was unusually extensive and connected to the white spot on P8, hence creating that very same "string of pearls" effect assumed indicative of Mew Gull.
The key to identifying it as an unusual Common was the pattern of P8 alone: Mew Gull should not show such an extensive black tip with a white spot but show a large white tongue at the base that goes all the way up to where the white spot on P9 is. The black on P8 should only be around the tip on Mew Gull.
Below are two heavily cropped images of the normal presumed western Common Gull (top image) already shown above and the strange Common Gull. Unfortunately I do not have a picture of a Mew Gull to complete this comparison. As a matter of fact, I have yet to see a Mew Gull (can you believe it?). But nevertheless, here's the comparison:
What can be seen with copious amounts of good will on these horrible images is that the basic pattern is essentially the same (hence very unlike Mew) but that the bird shown below simply has quite a lot of white where the bird above shows only some white.
To conclude this very long post:
According to my limited experiences along the Baltic coast, very few Common Gulls can show a wing tip pattern reminiscent of Mew if not seen clearly (e.g. on a distant bird in flight), and to secure any identification of a bird (be it as a Common or a Mew) one should focus strongly on the pattern of P8.
Isn't it amazing what a difference an inch of black on a single feather can make? What I would have done if this bird had a broken-off outermost primary I do not know... Stayed calm, counted the primaries and noticed the mistake? Yeah, I suppose so.