Lessons from Travels: Glaucous Gull Diet

by Carl Strang

This is a lesson in the biases we unconsciously insert in our observations. My Ph.D. thesis work in the 1970’s centered on the diet of glaucous gulls in western Alaska.

The glaucous gull is one of the largest gulls in the world, and it lacks the black or dark gray wingtips shown by many of its relatives.

I was hosted by the Fish and Wildlife Service, which was responsible for coastal refuges where abundant water birds breed. Their emphasis was on the reproduction of geese: emperor geese, white-fronted geese, cackling geese, and brant.

Emperor goose on the nest.

The waterfowl biologists who managed the refuges were aware of the whole range of species on those lands, but their interest and bias was so focused that they thought of other species mainly in their relationship to the geese. They noticed glaucous gulls mainly when they preyed on eggs or goslings, and so had developed the biased assumption that the gulls mainly ate geese.

Remains of a goose egg consumed by a predator.

The gulls are big enough to swallow goslings whole. Thus I was supported in my study of gulls. I started out on the Bering Sea coast, where there were several nesting colonies of glaucous gulls. My study area was at Kokechik Bay, at the very tip of the Yukon-Kuskokwim Delta. The Bering Sea is shallow, and when the tide went out, the bay became several square miles of mud flats with shallow pools. Only the central river channel remained.

Kokechik Bay, low tide, typical Bering Sea weather. The river channel is too far out to see from this low angle.

I found that when the tide went out, so did the glaucous gulls. The bay’s fish became concentrated in the river, and the gulls could be seen diving for them in the distance. As the tide rose, they walked ahead of the water, snacking on marine invertebrates stranded in the mudflat pools. On the coast, despite the large number of gulls, their impact on geese was limited to occasional, opportunistic predation of nests and unprotected young.

I went inland for my final field season, and found the story was different there. Instead of broad intertidal zones, relatively small areas were exposed as the tide rose and fell in the rivers and sloughs.

Inland study area, with a slough empty at low tide. I was able to make use of driftwood observation towers ingeniously constructed by Pete Mickelson, my predecessor on that site, who studied cackling geese.

Glaucous gulls were present only as widely scattered nesting pairs. Their diet had a significantly greater emphasis on eggs and young birds, but the impact was small because there were so few gulls.

This subject has been of sufficient interest that my study has been repeated a couple of times by others in the decades since, but expanded in terms of methods used and geographical area covered. They have confirmed my results, but there has been one new development. Glaucous gull numbers in western Alaska increased greatly after 1980, apparently because of improved winter survival thanks to scavenging from the growing human communities and an expanded commercial fishery. More gulls have meant more predation on waterfowl, to the point where population control measures have been considered. Now, though, instead of simply reflecting the biases of casual observation, the impact on waterfowl is assessed quantitatively, and takes into account the difference between coastal and inland diets. It is mainly the inland glaucous gulls that may be controlled.

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