by Carl Strang
A common pattern in parts of the world I have visited is the presence of 2-3 species of gulls: a large, usually white-headed gull, a smaller white-headed gull, and sometimes a still smaller black-headed species. All three were represented on my study area in western Alaska. There, the large species was the glaucous gull, focus of my research.
The small white-headed gull was the mew gull, a widespread species known in Europe as the common gull.
Finally there was Sabine’s gull, smaller than the other two.
In south coastal areas of Australia the largest species is the Pacific gull.
The place of the small white-headed gull is taken by the beautiful silver gull.
There is no small black-headed gull in that region.
In our part of the world, the most common gull is the ring-billed gull, one of the small white-headed species.
Herring gulls are our large white-headed species. We have no small black-headed gull in the Great Lakes area, but on the east coast there are laughing gulls.
While there are exceptions, and caveats, in many parts of the world this pattern holds among this group of generalist birds. Such observations have led to ecological theories of guilds (groups of similar species that subdivide a common resource) and niches (the ecological roles or jobs performed by different species). Here, body size does the job, and foods are the divided resource. In Alaska, glaucous gulls ate foods ranging from goslings and larger fish to voles and larger marine invertebrates. Mew gulls also took voles, as well as smaller fishes and bird eggs, insects and marine invertebrates. Like the glaucous gulls, they scavenged from dead animals they found. Sabine’s gulls focused more on smaller invertebrates and minnow-sized fishes. I don’t remember them as scavengers.