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On the Trails: Glaucous-winged gulls

Posted: January 20, 2012 - 1:01am
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A sea star is a big mouthful for a gull.  Courtesy of Bob Armstrong
Courtesy of Bob Armstrong
A sea star is a big mouthful for a gull.

Locally, the glaucous-winged gull is one of our most common gulls, and they are easy to watch, so I thought perhaps it was time that I write something about them. These gulls are bigger than the medium-sized mew gulls and the small Bonaparte’s gull, but about the same size as herring gulls, which are also quite common, and Thayer’s gulls, which come through on spring migration. As adults, however, they are easy to distinguish from herring gulls and Thayer’s gulls, because they lack the black wing-tips of those two species. The adjective “glaucous” means “gray.”

Young herring and glaucous-winged gulls are much harder to tell apart, but it can be done. These big gulls take three years to reach adult plumage and sexual maturity. So, they spend two years in immature plumages, which feature various shades of brownish gray but no black (check a good field guide!).

Thousands of glaucous-winged gulls attend the eulachon spawning run in Berners Bay in spring. They swoop and dip down to nab the weak-swimming fish, but they sometimes miss, leaving a fish with puncture wounds. They also scavenge eulachon stranded by an outgoing tide or dropped by other predators. In our study, adults were more successful at catching fish than immature gulls. Young birds gathered in small gangs on sand bars and chased more successful foragers in attempts to steal a fish. However, their attempts at pirating fish from other birds were not very successful, even when the victim was a smaller kind of gull. Indeed, the immatures were notably poor at pirating fish from other birds, and would have garnered more fish if they had caught fish from the river for themselves.

Other species of gull are there too, all gobbling up the fat-rich eulachon. Adults of all species were about fifty to 60 percent successful at diving for their prey. The big gulls could swallow the fish quickly, reducing the risk of another bird stealing the prey. But the little Bonaparte’s gulls often had trouble swallowing a fish, especially the larger male fish. The longer handling time meant that these gulls were more likely than the bigger gulls to lose their fish to a pirate.

All kinds of gulls gather at salmon runs in summer and fall, but they tend to forage in different ways. For example, at pink salmon runs in Juneau, adult glaucous-wings foraged more often on carcasses than did immatures, and immatures were more often seen feeding on loose and drifting eggs. These eggs were doomed in any case, because they were not buried in the gravels to incubate. Adults glaucous-wings occasionally pulled live salmon from the stream, poking initially at the eyes or at the vent area to force extrusion of eggs. Neither age class foraged much on intertidal invertebrates.

In contrast to the adult glaucous-wings, the medium-sized mew gulls fed mostly on invertebrates in the intertidal rockweed and to a lesser extent on salmon eggs. The small Bonaparte’s gulls foraged mostly on eggs, and less frequently on intertidal invertebrates.

In winter, there are often quite a few glaucous-winged gulls in Auke Bay and the downtown harbors. There, they forage on whatever they can catch, including larval fish, shellfish, and sea stars. A sea star seems to be mostly bony plates and very little soft tissue, so I wonder just how much nutrition a gull can extract from eating one. I also wonder if some of the sea stars that are missing one or two arms might have been assaulted by a big gull. Sometimes, however, these gulls are reported to swallow whole sea stars; this process apparently takes a considerable time.

Opportunistic foragers, the big gulls can also be found picking bits of meat off discarded deer carcasses on the beach and scavenging whatever looks edible at the dump. Sometimes they pirate mussels from scoters, and swallow them shell and all. They hang around our grocery store parking lots with the ravens, hoping for a handout or some goodies in the back of a pickup truck.

• Mary F. Willson is a retired professor of ecology.

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