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Most people have strong opinions on seagulls, the generic term for the local gulls begging outside many seaside fast-food outlets. Some think of them as charismatic native birds; others as rats of the sky. Throwing them French fries and bread is fun and makes us feel like we are doing the birds a favor. But are we really?

I have always been concerned about our impact on wildlife and this led to my doctoral research on how human-sourced foods affect gulls, in this case the local Silver Gull, the common seagull found throughout most of Australia. In most major Australian cities, the range and population of these birds have expanded and their increasing numbers have not endeared them to us, especially as they harass people for food, defecate copiously in public areas and may provide a pathway to diseases.

We have only ourselves to blame. Gulls are generalists and opportunists, meaning they are ecologically primed to take advantage of any additional foodstuffs. Human habitation, with our fast-food outlets, garbage dumps, picnic areas and overflowing trash cans, provide them with plentiful, easily accessible grub. I’ve termed this diet ‘garbivory.’

Measuring the Effects of Garbivory
Do the junk-food diets of urban gulls affect their health in the same manner as they would humans? I devised experiments that measured potential effects on chemical, biochemical, physiological and reproductive levels. My control birds were those found on the isolated Furneaux Islands, where they tend to eat natural fare—mainly insects, crustaceans, worms, fish and berries. Experimental birds were from Hobart, the capital of Tasmania. These gulls tend to exploit our leftovers; in their pellets and regurgitations I recognized cooked chicken, hamburger, ham, bacon, French fries, white bread, pasta, dog and cat food and even cigarette butts.

Gulls can fly long distances but rarely do so during their breeding season, when they are also easier to trap in their colonies. However, I needed to discern that the remote and urban sites were separate feeding regimes. Isotopes, or atoms of the same element with different numbers of neutrons, were the answer. My analysis of stable isotopes in their blood suggested that remote gulls fed on items from a more marine origin, while urban gulls fed from a separate food web and from a more freshwater origin.

The populations were then considered distinct. But did the easily accessible, predictable and abundant foods make the urban gulls heavier? A short flight to the local dump would provide all their daily food within minutes, leaving the day free for loafing. After weighing and measuring gulls from each population, I found that the urban birds were significantly heavier—by 10%—than their remote cohorts even through gulls from both populations were exactly the same size structurally.

Normally in wild animals and birds in particular, heavier means fitter. More body mass means more resources to spend on important activities like defending territories, laying eggs and flying. I wasn’t confident that these heavier urban gulls were healthier, given their insalubrious diet, so I investigated from a different angle—blood biochemistry.

People get blood tests to provide valuable insights into health and nutritional status. Would gulls, like humans, show increases in cholesterol, triglycerides and glucose as a result of this meaty, starchy menu? My data indicated that the garbivorous gulls had greater levels of cholesterol and generally higher levels of glucose than remote gulls.

Given that the garbivorous gulls were eating distinctly different diets, were heavier and showed disconcerting blood-test results, I needed to confirm if this impacted their ability to raise chicks. My research showed the eggs from remote nesting gulls were significantly larger and heavier. They also had greater yolk mass and higher carotenoid concentrations compared to eggs from urban areas, which suggests greater reproductive success. Although urban gulls were successful laying eggs, poorer hatching and fledgling success may have resulted from smaller, lighter eggs that contained less yolk and fewer carotenoids. The urban gulls were also laying eggs that were significantly smaller overall than those collected over the past 100 years. What this means is that urban gull chicks may not be as physically robust as those from more remote locales.

What’s the Super-Sized Picture?
Although each of these queries deserves additional research attention, the garbivorous diet of the urban gulls may manifest into harmful health outcomes, much as they would in us and our pets. Many complain that seagulls are superabundant pests; however, they are merely opportunistically eating what we provide. I believe these native birds are a valuable indicator species, reflecting the health of our urban ecosystems. Those potentially negative consequences are a disturbing reminder that what we do to ourselves extends to the natural world.

Additional resources:
H.J. Auman dissertation (pdf)

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