Bird's Eye View

What impact do people have on seagulls?

This seems like a straightforward question, and in the January issue of The Condor: Ornithological Applications, three researchers set out to answer it. They compiled an amazing 111 years of population data for glaucous-winged gulls in the Georgia Basin of British Columbia, Canada, and then tried to explain the bird’s ups and downs (there were a lot of them).

Lead author Louise Blight, now a senior scientist at Procellaria Research & Consulting, says gulls have a lot to tell us about how marine ecosystems respond to change, both natural and human-fueled.

Gulls are nothing if not hardy. They are opportunistic carnivores, which means they hunt other birds, fish, mollusks, mammals, and basically anything else that will fit down their gullet—which is pretty much everything, since they can unhinge their freaking jaws.

But even the most successful predator can have its day rocked by unruly neighbors—especially when those neighbors come over to its island and steal its eggs. And that’s where the data and our story begin.

Leggo My Eggo

By poring over documents from around 1900, Blight and her coauthors found that the practice of hunting seabird eggs was once very common. Canada's First Nations people prized gull eggs as an important subsistence food, and soon other Canadians and Japanese immigrants got in on the game, too. Whatever the egg hunters couldn’t carry, they smashed, to stimulate the gulls to lay more eggs for next time.

The gull population during this period was the lowest on record, and had egg-collecting continued, the species might have disappeared from the region. Luckily for the birds, humans giveth as well as taketh away.

In 1918, the United States and Canada passed a landmark piece of legislation: the Migratory Bird Treaty Act. One of the first environmental laws of its kind, the treaty greatly reduced egg-collecting and gull-hunting in the study area. Not surprisingly, Blight found that gull numbers started to rebound after 1920.

But the Migratory Bird Treaty might not deserve all the credit. At about the same time, the Georgia Basin experienced a growth in its human population. And with people came garbage.

The effect trash has on gulls is complicated. Some research, like another paper published in The Condor, finds that glaucous-winged gulls with access to garbage have higher fledgling rates. Perhaps this is because reliable food sources, such as landfills, allow adults to spend less time foraging and more time protecting their eggs and chicks. Or it could be that having an additional food source means gulls don’t have to cannibalize each other’s eggs. Or perhaps trashy treats can be high in calories, and well-fed parents make for healthier offspring.

Wait, there’s another possibility. Another body of research comes to a very different conclusion, pointing to the difference between calories and nutrition. Let’s just say gulls eating leftover donuts and casserole may not be getting everything their bird bodies need. Indeed, other studies indicate that gulls that eat forage fish lay larger eggs and larger clutches; gulls supplemented with protein produce larger and heavier hatchlings; and gulls who ate more fish had better reproductive success.

Trading Fish for Pesticide

By far, fish are a gull’s best diet—and thanks to humans there are fewer fish around. In another study last year, for which Blight was also lead author, researchers analyzed isotopes found in 150 years worth of glaucous-winged gull feathers. Among other things, the paper showed that the bird’s diet has become significantly less marine since 1860.

Take eulachon, which is a fancy name for smelt. Historical records from the 1940s document show how glaucous-winged gulls would gorge themselves on eulachon during the fish’s spawning season. But smelt spawning season isn’t what it used to be. Hydroelectric dams, dredging, sedimentation, climate change, and pollutants have led to sharp population declines for the fish. In 2010, the National Marine Fisheries Service designated eulachon as “threatened.”

I know, we started off with gulls and now we’re talking about smelt. The can is open; the worms are everywhere. But if you can stand one more twist, I’d like to tell you about how DDT might have been good for the gulls.

Everybody knows the DDT story—popular pesticide, one of the lead villains in Rachel Carson’s Silent Spring, finally banned in 1972. Among its many other sins, DDT ravaged bald eagle populations by thinning their shells.

Eagles eat gulls. The raptors also raid gull nests and force the parents to spend energy standing guard. So anything that reduced the population of eagles—even an environmental scourge—was a boon for the glaucous-winged gulls. It’s basically an enemy-of-thy-enemy-is-thy-friend situation. Our ripples have ripples.

Ups and Downs

Whether it was the eagle downfall, the free garbage, the international bird treaty, or some combination of the three, glaucous-winged gull populations rose steadily from 1920 to the '80s. And then they took a turn for the worse—perhaps due to the bald eagle comeback or the continued disruption to forage fish populations.

Today, the glaucous-winged gull population sits at just 50 percent of its peak level. Blight says it’s difficult to know what that number means, given that we don’t know how many gulls would have inhabited this area had humans never showed up. Maybe those peak numbers—perhaps during those good garbage years—were only possible because humans did come on the scene.

“As a society we try to address the environmental impacts of our various activities, but how successful can we be at mitigating impacts or maintaining healthy ecosystems if we have a crippled or distorted sense of what is normal?” Blight asks.

That’s why research like this is so important. It may not reveal a smoking gun, but it demonstrates how our actions have the ability to send shock waves through the environment, with consequences that sometimes take decades or centuries to reveal themselves. If we don’t have good baselines, we can’t accurately measure our influence on the world around us. And if we can’t measure impacts, there’s no way to know, for instance, how drilling for tar sands will influence caribou numbers, or how highways hurt the Florida panther population, or what effect pesticides may have on migrating monarchs.

So the story of the glaucous-winged gull is also the story of bald eagles and eulachon and countless other species in the food web. Humans are in there, too, of course. Sometimes we just don’t act like it.