Gulls, often generically called “seagulls,” are successful seabirds defined by their highly flexible and opportunistic feeding behaviors. Belonging to the family Laridae, they are recognized as generalist feeders with an omnivorous diet that shifts based on availability. Their remarkable adaptability allows them to thrive across diverse environments, from remote coastlines to bustling inland cities.
The Coastal and Marine Diet
The natural diet of gulls is rooted in the marine environment, primarily consisting of small fish and invertebrates. They commonly consume crustaceans (such as crabs and shrimp) and mollusks (including mussels and clams), often extracting them from intertidal zones. Gulls employ specialized foraging techniques, such as “foot-paddling” to stimulate prey movement in shallow water, or dropping hard-shelled prey onto rocks to access the soft interior.
Beyond the shoreline, their diet expands to include terrestrial prey, especially during the breeding season. Gulls consume insects, earthworms, and small mammals like rodents. They also scavenge carrion, including the remains of larger marine animals washed ashore. Furthermore, they are opportunistic predators of unguarded eggs and chicks from other bird species.
The Urban and Anthropogenic Diet
Gulls have successfully colonized urban areas by exploiting the abundant, predictable resources produced by human activity. Their role as scavengers is most visible at municipal landfills and waste disposal sites, which provide a reliable, year-round food subsidy. Here, they consume discarded food items, including bakery products, fast-food scraps, and general refuse.
This shift in diet involves a high intake of processed foods, such as wheat-based items like bread, and agricultural byproducts like discarded grain. The gulls’ behavioral flexibility allows them to adapt foraging schedules to human patterns. They often congregate at waste centers when they open or anticipate school break times for easy access to discarded snacks. Some urban gulls have demonstrated a learned preference for food items they observe a human handling, associating human action with an obtainable meal.
Why Human Food Sources Pose a Problem
The reliance on human food discards, known as anthropogenic food, introduces significant biological and ecological ramifications for gull populations. While human refuse is high in caloric value, it lacks the balanced nutritional profile found in their natural marine diet. A diet dominated by grain or processed food can be severely deficient in calcium, a nutrient vital for strong bone development in chicks.
The consumption of high-fat, high-salt scraps can lead to physiological stress and elevated cholesterol levels in the birds, compromising their long-term health. Ecologically, the abundant food supply in urban centers supports unnaturally high population densities, leading to increased competition and aggressive behaviors, such as food-stealing from people or other birds. This dietary shift can also lead to an “ecological trap,” where gulls become so habituated to the readily available urban food that they lose the ability to exploit healthier, natural food sources.