A possible hybrid Herring Gull (Larus smithsonianus) x Lesser Black-backed Gull (Larus fuscus)

South Harwich, Massachusetts

23 February 2008

Blair Nikula

The adult gull illustrated in the images below was photographed at Red River Beach in South Harwich, Barnstable County, Massachusetts under overcast conditions on the afternoon of 23 February 2008.  It was among a concentration of 250+ Herring Gulls, 125+ Ring-billed Gulls, and a few Great Black-backed Gulls feeding in the wrack line, and was observed at distances of ~100-150 feet. It was present through at least 1 March.

I initially thought this bird was a good candidate for Yellow-legged Gull (Larus michahellis), but some features are wrong for that taxon (see discussion below), and the majority opinion (including mine) is that is more likely a hybrid Herring x Lesser Black-backed Gull.

All images were taken with a Canon 20D camera and 100-400 Canon IS lens.  Click on the thumbnails for full-sized images.
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The first three images below show the wing pattern very well, the first two the upper surface and the third the under surface (in direct comparison to a Herring Gull in front of it). The fourth image shows the gape.

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The bird seems to me to be a pretty good fit for an adult Yellow-legged Gull in most respects. Salient features included a mantle that is intermediate in tone between an adult Herring Gull and an adult graellsii Lesser Black-backed Gull.  (In many of the photos, the mantle appears as dark as a graellsii, but at no time during the 45+ minutes I watched the bird did it look that dark to me.)

The legs varied in appearance, depending upon the light and angle, from yellow, to orange-yellow, to pinkish-yellow (and this variability is apparent in the photos, though the legs looked brighter in the field than they do in the photos), and were much brighter than the legs of any of the Herring Gulls in the area.  It was similar in size and structure to the largest Herring Gulls in the area, thus larger and huskier in build than a typical Lesser Black-backed. The bill is also quite large and thick, and rather unlike a Lesser Black-backed.

The primary pattern seems right for Yellow-legged, at least for the most part: primaries overall more extensively black than on a Herring Gull (but pretty similar to a Lesser Black-backed), with a fairly large white mirror on P10, only a tiny white spot on P9, and a black sub-terminal bar extending across the width of P5. One potential problem with the primaries is the extent of black on P7 & P8, which perhaps should be more extensive (i.e., extending farther into the base of the feathers) in a Yellow-legged. In this respect, the pattern seems more similar to a Lesser Black-backed. The white mirror on P10 is also a bit larger than typical for Yellow-legged, though probably within the range for that species.

The orbital ring was reddish, though not discernibly different from some of the Herring Gulls present. The gape was reddish (evident in the last flight shot above). The dusky patch near the tip of the bill may be atypical for an adult Yellow-legged Gull, and the tip of the bill is more pointed (or less rounded) than it should be.

The brown streaking on the back of the neck is another - and perhaps most significant - issue. From all accounts, this is not at all typical for an adult Yellow-legged Gull in basic plumage. And Yellow-legged Gulls are supposed to molt into alternate plumaged earlier (on average, at least) than Herring or Lesser Black-backed and should be very white on the head (and neck) by late February.

So, the main sticking points with regards to this being a Yellow-legged Gull seem to be the extent of black on P7 & P8 and the brown streaking on the nape. The dusky on the tip of the bill, the bill shape, and the leg color (not as bright and yellow as expected on an adult Yellow-legged at this season) may also be issues.

As has been noted in similar discussions in the past, a bird that is far out of its normal range, may not molt on a normal schedule, and may not have normal hormonal levels, which could lead to atypical soft color parts (bill, legs, orbital ring).

To muddy the picture further, there are multiple forms of Yellow-legged Gull, the exact number varying depending upon whose taxonomoy you accept, which differ from one another in some of the same features in which the Yellow-legged taxa differ from other similar large gulls (i.e., mantle color, primary pattern, body structure).

The Harwich gull perhaps could be an extremely large, pale Lesser Black-backed, but at no time while watching this bird in the field has that seem like a viable candidate to me, nor have I received any feedback arguing in favor of that species. A more likely possibility to my mind would be a hybrid Herring x Lesser Black-backed, though the size and structure seem extreme for that pairing.

The bottom line is probably that, despite being about as well seen and photographed as a gull could be, and despite knowing what virtually every feather on the bird looks like, the identity of this bird may well remain mired in uncertainty. So it goes with large gulls!

Thanks to Rick Heil, Dick Veit, Marshall Iliff, Julien Hough, Nick Bonomo, Bruce Mactavish, Jared Clarke, Louis Bevier, and Bob Wallace for their comments and input. Should anyone have any additional insight or opinions to offer, I will be pleased to here from you.

Blair Nikula