A hybrid Great black-backed x herring gull at Girdle Ness

 

 

On the morning of January 21st, 2018, like every other day, I took my cup of tea to the office window overlooking Aberdeen harbour, to see what I could see. Most of the time there’s nothing of note, but occasional otters, divers, grebes, white-winged and mediterranean gulls make it a habit worth indulging. On this particular morning, I noted nothing remarkable immediately, but after a moment or two my eye was drawn to a rather puzzling gull. 

 

Even without binoculars this gull seemed something of a contradiction. The colour of the mantle suggested a lesser black-backed gull, but the rest of the bird was not right. It was January after all, and lesser black-backs are very uncommon in Aberdeen at this time of year. On top of this, it was a pretty hefty creature, being slightly bigger and heavier than the herring gulls around it. Through the binoculars, other features didn’t fit. At a distance, the head looked unstreaked and the legs were pale pink. It was obviously time to abandon the idea that it was a lesser black-backed gull.

 

I now had great black-backed gull and slaty-backed gull as options, so I grabbed my camera and ran down to the shore in front of the office. I managed to get a couple of shots of the bird on the ground and then kept my finger on the shutter as the bird took off. A quick review of the primaries on the back of the camera showed no ‘string of pearls’, confirming my impression that this was not a slaty-backed gull. Unfortunately.

 

So this left me with the palest great black-backed gull I’ve ever seen in my life and a rapidly cooling cup of tea, and with the bird gone I returned to my desk. However, the paleness of the mantle was really niggling me and I was beginning to wonder if I needed to think about a hybrid.

 

I had to wait for an opportunity to have a proper look. It taunted me for the next couple of days, perching up on the streetlights in the office carpark on my way into work. Even in the half light it looked pale, but by the time the day had properly broken the bird was nowhere to be seen. When I did finally get a decent look (and some decent photos), a number of features that did not fit great black-backed gull were detected. These are discussed below.

 

Mantle: As discussed, this was pale. Perhaps not pale enough to rule out a pure GBBG entirely, but always strikingly paler than the others present, on the odd occasion that they were seen together. The mantle tone varies in photographs but the relative paleness compared to a GBBG was consistent.

 

Head streaking: Most GBBG show no, or at least very little in terms of head streaking in winter. The streaking on this bird was light, but more prevalent than on a standard GBBG.

 

Eye ring: Most GBBG show a rather bright red eye ring. Close views of this bird showed the eye ring to be a rather pale browny orange.

 

Primary pattern: Three elements of the primary pattern were anomalous with what would be expected for a standard GBBG. The most obvious of these was the strong contrast between the black of the outer primaries, and the grey of the rest of the upper parts. Closer examination of the wingtip revealed that p5 had a broken black band near the tip, and that p10 had a small black spot near the tip.

 

Structure: Although the bird was clearly bigger than a herring gull when the two could be compared, it lacked the obvious bulk of a GBBG, especially around the head and bill, and appeared to have a more attenuated rear end.

 

I suspected that none of these features individually ruled out a GBBG. To assess how frequently each feature occurred in adult GBBG around January, I looked through images of GBBG taken from December to February on Gull Research website (http://www.gull-research.org/). The proportion of birds showing head streaking, an orange eye-ring, and black on p5 and p10 was assessed (i.e. the number showing each feature from the number of individuals from which each feature could be assessed). The proportions are shown in the table below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Without knowing whether the occurrence of any of these features are linked to one another it is only possible to speculate on the likelihood of all of these uncommon features occurring on one bird. However, it would seem extremely unlikely, especially in combination with the structural differences and the pale mantle (neither of which were assessed as I could not come up with an objective way of doing it). As such, it seemed that a hybrid, most likely with herring gull, was the most likely answer. Posting images onto various facebook groups for Larophiles returned a number of different suggestions but the most numerous was for a GBBGx HG hybrid. 

 

Hybrids between GBBG and herring gull are very uncommon in Europe, but are much better known from North America, where they are labelled ‘Great Lakes gull’. Images generally show these birds to be paler than the bird at Girdle Ness. Another feature potentially at odds with a GBBG x herring gull hybrid is the amount of black on p5. Most herring gulls show no black on P5, with this feature most likely to occur in populations from further north and west (which, incidentally, would have darker upper parts than local birds as well). Other potential parent combinations could account for these features (e.g. GBBG x lesser black-backed or yellow-legged gull) but these seem even more unlikely and neither account for the full suite of features seen on this bird.

 

It was last seen on January 31st, when I managed to tempt it in close enough (with a packet of oatcakes) for some decent photos. Some of these, annotated to show the key features, are included here.