It was already evening at the outlet of the Gülper See into the Havel. The air was filled with the calls of migrating water birds. A white-headed gull sat on a boat bollard. In addition to the familiar sight of swans and ducks, this gull caught my attention. The first thing I noticed were its pink legs and feet. The common white-headed gull at the Gülper See is the Caspian Gull (Larus cachinnans), which can be found in dozens at the Gülper See, especially in late summer and autumn. Something about its posture and overall appearance, in addition to the color of its legs, seemed to be off. When I pointed my binoculars at the gull, the feeling grew that I might be looking at a Herring Gull (Larus argentatus). To be on the safe side, I took a few photos. The analysis at home then confirmed another feature, which was more in keeping with the Herring Gull than with a typical Caspian Gull. Or could it be a crossbreed?
The first clue to this gull’s potentially mixed ancestry was in the look of its head. The head was not quite typical for a Herring Gull, lacking the more angular, squarish shape often associate with adults of the species in literature. This gull’s head was more elongated, resembling the sleeker profile typical of Caspian Gulls, yet something seemed amiss. While a Caspian Gull typically has a more delicate and refined bill, this bird’s bill was thicker and bulkier, a trait more characteristic of Herring Gulls. This intermediate bill structure suggested the possibility of hybridization.
The coloration of the bill also presented an interesting mix of traits. It lacked the all-black, sharply defined markings often found on a pure Caspian Gull, yet it did not perfectly align with Herring Gull features either. Although bill shape and structure are important in identifying gulls, they alone do not tell the whole story. The lack of a fully pale iris, a common characteristic of adult Caspian Gulls, further complicated matters. Caspian Gulls with pale irides do exist, but this bird’s iris was not completely pale, again suggesting some degree of mixed lineage.
Another characteristic that pointed toward hybridization was the absence of extensive head streaking. Pure adult Herring Gulls typically exhibit pronounced streaking on the head, especially during the winter months. In this bird, however, the head remained relatively clear of such markings, a feature more consistent with Caspian Gulls, which generally show little to no streaking.
This lack of head streaking, combined with the intermediate bill structure and the solid P5 mark, suggested that the bird was not a pure Herring Gull. Yet, the overall bulk of the bird, especially in its body structure, seemed too robust for a typical Caspian Gull, leaning once again toward the Herring Gull side of the spectrum.
As any experienced gull watcher will know, the pattern on a bird’s spread wings can reveal a lot about its identity. In this case, however, the opportunity to capture a spread-wing shot proved difficult as the bird quickly fled when approached and no flight shots were possible. One feature might have been noticed then, namely the continuous, broad black marking on the flight feather, known as the P5. This is not commonly seen in purebred adult Herring Gulls, which usually have a narrower P5 marking or even lack it altogether. In contrast, a conspicuous P5 marking is regularly associated with Caspian Gulls.
Identifying large gulls can be a notoriously difficult task, especially when dealing with possible hybrids. While an open-wing shot could have provided more conclusive evidence, the combination of features observed – the solid P5 mark, the lack of head streaking, the mixed iris color, and the intermediate bill structure – makes a strong case for this being a putative Herring x Caspian Gull hybrid.
As always in the world of birding, not every individual fits neatly into our field guides. In cases like this, careful observation and a nuanced understanding of species variation are key to unlocking the mysteries of avian identification. Whether hybrid or not, this enigmatic gull added a new layer of intrigue to an already fascinating day at Lake Gülpe.
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