Since the last post was quite long I thought it better to devote a separate post for this topic. In this post I will cover the separation of Yellow-legged Gull from the very similar Herring X Lesser-Black-backed Gull hybrids. For the purposes of this discussion I will be focusing only on adult birds and the comparison will be made between atlantis subspecies Yellow-legged Gulls and Herring (unknown ssp) X Lesser-Black-backed Gull (presumed Graellsi)hybrids.
This problem of HERG X LBBG hybrids is becoming increasingly relevant as the numbers of LBBG's in North America continue to increase. We have already had several instances of hybridization between graellsi LBBG's and Smithsonianus HERG's. A very well documented example and some nice discussion by Kirk Zufelt can be found here The Expansion of LBBG Some discussion about the discovery of a LBBG paired with a HERG on Appledore Island NH an be found here Appledore Gull As well,there have been several documented insyances of suspect LBBG X HERG hybrids getting friendly with Herring Gulls in Labrador,but no evidence of mating. The issue of HERG X LBBG hybrids is problematic enough, when you consider what a LBBG X HERG paired with a HERG would produce....well I don't even want to think about it!
Anyway, I'm getting off track , the purpose of this article is to discuss the separation of atlantis YLGU's from HERG X LBBG hybrids, so lets get started on that. I'm going to compare Yellow-legged Gull and HERG X LBBG hybrids based on their main points of separation, namely, mantle color, winter head streaking, bare parts coloration and wing tip pattern. I will not make any inferences about moult timing, since for hybrids,this is at best poorly understood,but from my experience most HERG X LBBG hybrids seem to follow a predominantly LBBG type moult schedule.
This is often the first thing I look for when searching for YLGU's among mixed gull flocks in St.John's. When I'm guiding and I can't find a YLGU for my client I will sometimes seek out a LBBGXHERG hybrid to show them the mantle color we are looking for. This works great because atlantis YLGU's and HERGXLBBG hybrids seem to have identical mantle shades.
Not surpriseingly hybrid HERG XLBBG's have a mantle shade that is pretty much perfectly intermediate between those two species. Incidentally, atlantis YLGU's happen to shade that very same shade of gray. One small difference may be that the hybrids can look a little bluer. That could just be the way I see them.
Winter head Streaking
One of the most well known feature of YLGU's are their tendency to be purely white headed in winter. They are usually not free of the last remnants of winter head streaking until early January, although it's probably possible in December. This is when separating YLGU's from potential hybrids is much easier because the potential pitfall hybrid combos will have head streaking lae into winter and some even early spring. I saw two hybrid LBBG's yesterday and both still had extensive head streaking while the YLGU near by was completely and beautifully white headed and has been that way for almost two months.
HERG X LBBG hybrid- Photographed St.John's Jan 28th
Note the extensive head streaking, even extending down onto the breast. This manner of head streaking would never be possible in YLGU,especially so late into winter.
YLGU- Photographd St.John's Jan 30th
This is what every YLGU should look like by the second or even the first week of Jauary. If there are any remnants of winter head streaking it will be restricted to the lores and forehead. After the first week of jan any YLGU with extensive Head streaking, isn't a YLGU!
In winter separating YLGU from LBBG hybrids is relatively straightforward. I have never seen a LBBG hybrid show bright yellow legs in winter. In summer all bets are off, the legs get scary bright on these things, but in winter they tend to show at best yellowish or maybe straw colored legs. Often times the legs appear flesh colored with yellowish spots, mainly around the knee joints. By contrast YLGU's ALWAYS have yellow legs. That is except for a very small percentage of apparently fleshy colored individuals. All the YLGU's we've identified in NL, have had nice yellow legs in all seasons, getting brighter as winter progresses. In my experience they are NEVER yellowish, pale yellow, greenish or any other permutation of yellow. If you see a YLGU that doesn't have pure yellow legs, it probably isn't a YLGU!
Note leg color and mantle color compared to immature LBBG to the right. Photographed at St.John's Feb,2009.
In my experience this is about as yellow as LBBG hybrid legs get in winter. These will probably be bright yellow in late spring.early summer. Photographed in St.John's Jan 2009.
Another example of standard leg color for HERG X LBBG hybrid.
Here is YLGU showing probably as dull yellow legs as you will see. Note while they are a dullish yellow, they are still a pure yellow without any greyish, greenish or flesh tones. These become bright yellow by mid December. Photographed in St.John's mid October 2010.
Aside from leg color, there are also differences in orbital ring color and to a lesser extent bill coloration. LBBG hybrids commonly show an orangy yellow or orange orbital ring, while YLGU orbitaj rings are usually deep red. As stated in the previous article, in early fall when at the height of basic plumage some YLGU's can show an orangy red orbital ring. Therefore its important to consider the date of obsevation when separating YLGU's from potential hybrids.
Note: Standard lookng head of HERG X LBBG hybrid in mid winter. Lots of head streaking, extra concentrated around the eye. Orange orbital ring,pale, clear eye and bill with decent sized gony spot with some subterminal black markings in the bill. In my expereince most LBBG hybrids show these black markings in the bill in winter, while only a minority of YLGU's will.
Typical YLGU from late December onwards.Pure white head, deep crimson orbital ring,large red gonys (does not need to bleed into upper mandible), clear, ple eye and clean bill, lacking black markings.Many will also show an extensively red gape line.
Even when streaking headed in latesummer and early fall YLGU's will never show streaking onto the breast and lower nape lik most HERG X LBBG hybrids do.
Separating YLGU's from possible hybrids based on primary pattern is not exactly strightforward, especially since they can show the same general pattern of black on the outer 6 primaries, p5-p10, with a single mirror on p10 only and a black band on p5. Upon closer inspection though there are some differences to be noticed.
While generally quite close to YLGU, ths wing tip shows several differences. Notes especially the reduced black on p8 and the grey tongues eating into the blakc on p6 and p7. As well,there is extremely minimal black on p5, barely forming a thin broken band on only one wing. That smal mirror on p9 on the right wing also is a strike against YLGU.
This wing tip is closer to YLGU than the one above but notes especially the reduced black on the inner webs particularly on p6 and p7. Even more important are the pale moons lie between the blak and gry on p5-p7. These are not seen on YLGU. This is pretty sibtle though and you'd probably want a photo to show them.
Pehaps the most YLGU type wing possible in a HERGXLBBG hybrid. In fact this primary pattern is probably inseparable from msot atlantis YLGU's in the field. There are a couple of differences to note- based on my experience. The pale moons on p5-p7 are still there, much rduced, but still visible. As well,the black sub terminal band on p1 is considerable. In my experience out YKGU's generally show a thinner subterminal band on p10. Ofte when our YKGU's show a subterminal band this thick on p10,they will also have a black mark on p4. Compare teh above wing tips with the YLGU wings below.
Note the thin subterminal band on p10 (no black on p4. Compare p5-p7 with the above birds, notice the lack of pale white moons where the black meets the grey areas on the primaries.
Note the thicke subterminal band paired with black mark on p4. Also lacking pale moons p5-p7. The blakc on mark on p4 is critcal here. According to Olsen and Larssen only %25 of atlantis have a mark on p4. I have never seen a HERGX LBBG hybrid with a mark on p4. Doesn't mean it can't happen, but it's probably pretty unusual.
When identifying YLGU out fo range and especially when ruling out hybrid combinations it's critically important to consider multiple characters and avoid jumping to conclusions based ona singe trait such as leg or mantle color. As you cna see above there is some overlap between YLGU and HERG X LBBG hybrids in many traits and one should be very careful to first rule out this hybrid combination before claiming any YKGU out of range.