Non-birders, alas, dismiss them as “seagulls” and rank them among the foulest of fowl. But among birders, gulls, especially the larger gull species, are often objects of particular fascination. Characterized by identification challenges, interesting biology, and formidable powers of flight and predation, gulls are part of birding’s graduate-level curriculum.
To understand what all the fuss is about, let’s look at one particular species, the lesser black-backed gull (“LBBG,” in birder shorthand). Primarily a bird of western Europe, this gull has gained a solid foothold in North America and represents everything that is interesting about gulls.
In general — and I’ll get to the “however” part in moment — this species is pretty easy to recognize. Its size averages marginally smaller than the herring gull, which is our most common and familiar gull species, and smaller by a good third than the great black-backed gull, which is the largest gull species in the world. In adult plumage, the LBBG’s eponymous charcoal gray back and wings are unambiguously darker than the same parts on a herring gull, and lighter than those parts of the great black-backed. And the LBBG is generally lighter in build and slightly elongated compared to the two larger species.
The various immature plumages that a large gull passes through for four or five years as it grows up are a bit more challenging. But overall, the rule of intermediate gray coloration holds true, and the attenuated structure of an LBBG is always a reliable mark once you get the feel for it.
However … LBBG as a species varies quite a bit, with three generally recognized subspecies that differ mainly in how dark their backs are. While the palest of these comprises the majority of the birds that turn up in New England, birds from other populations are possible here, given the nonchalance with which gulls traverse long distances. And even worse, the lesser black-backed gulls in North America appear to be interbreeding with herring gulls, the resulting occasional hybrids intermediate in coloration between typical herring gulls and LBBGs. It’s often hard to tell exactly what you’re looking at.
Here’s the history. As recently as the 1980s LBBG was a notable rarity in our region, with the odd individual making it here from Europe. But for reasons that are not entirely clear, the populations in northwestern Europe began growing rapidly and expanding to the west — to Iceland, Greenland, and ultimately the coast of New England, where hybridization of herring gulls and LBBGs has been noted on Appledore Island, off southernmost Maine.
Despite dedicated searching, I didn’t see an LBBG in Massachusetts until 1997, shortly before I moved to the Vineyard. But in the two decades since, numbers of this species have exploded in North America (though no one really knows why). I’ve seen 35 at a time on the south shore of Nantucket, and a flock of more than 100 (mostly immature birds) from a boat south of the Vineyard; records, often of large numbers of individuals, now span the continent.
Clearly these are not all long-distance vagrants, staggering in disoriented after an exhausting flight from Europe. Some, surely, are wanderers from the newly established Greenland population. But it seems clear that LBBGs are breeding somewhere in North America — likely several somewheres — and people just haven’t found those sites. An even more interesting question is how this species is interacting with herring gulls. To what extent are they hybridizing? To what extent are they in competition?
More broadly, the enigmatic breeding biology of the LBBG is representative of that of large gulls as a group. A suite of populations ringing the globe at high temperate and low arctic latitudes may almost be thought of as one single, gigantic species, varying both within and across regions. In one illustrative case, Thayer’s gull, regarded for decades as a good species that essentially filled the gap between the darkest Iceland gulls and the herring gull, was recently downgraded to a subset of the Iceland gull.
The concept of “species” isn’t quite adequate to characterize the relationships within this group. Closely related to one another, and moving freely around the globe on their powerful wings, these birds can readily interbreed or establish new colonies remote from the point of origin of the individuals that live there, gradually diverging from their ancestors in response to a different set of evolutionary pressures. Untidy, perhaps, but fascinating — more so than any other group of birds, the large gulls let us see population dynamics and even evolution play out over a time frame that we can actually grasp.
This has been a better than average season for LBBGs on the Island; somewhere between a handful and a dozen were reliably present through much of the fall along the South Shore, and several birds have been observed in association with recent storms. So keep an eye peeled for oddly dark herring gulls (or oddly small great black-backs?). This is a species on the move.