Gulls, often mistakenly called seagulls, are highly successful seabirds found across the globe, not just near the ocean. Their survival is linked to their status as opportunistic omnivores, allowing them to exploit diverse food resources from marine, terrestrial, and human environments. This adaptability is the primary reason for their widespread presence and population success, enabling them to shift their diet based on local availability and seasonal changes.
Natural Prey and Coastal Foraging
The core of a gull’s diet in the wild consists of naturally occurring food sources found in coastal and terrestrial habitats. Along the shoreline, they regularly consume marine invertebrates, such as barnacles, mussels, and crabs, which they often drop onto hard surfaces to break open their shells. Smaller gulls, in particular, are adept at hover-dipping or plunging into the water to catch small fish, though many species will also consume fish that are already dead.
Beyond the immediate coast, gulls exploit a variety of terrestrial resources, a foraging behavior that often increases during the breeding season. They search for terrestrial arthropods like earthworms and insects, which can be found in fields and mudflats. Larger gull species, such as the Great Black-backed Gull, are also capable predators, consuming small rodents and the eggs and nestlings of other bird species.
The composition of a gull’s natural diet changes significantly with the seasons and local availability. During the non-breeding season, marine prey makes up a larger proportion of their food intake as gulls spend more time over large bodies of water. Conversely, when establishing breeding colonies, they rely more on terrestrial prey to meet the demands of feeding young.
Scavenging and Human-Provided Food Sources
The presence of human activity has introduced a significant new category of food for gulls, moving them from natural foraging to opportunistic scavenging. A major source of this human-associated food is the waste generated by coastal industries, such as following fishing boats to consume discarded bycatch and offal. Gulls also frequently forage at open-air landfills and waste disposal sites, where they consume human refuse that is otherwise unavailable in natural environments.
Scavenging extends to urban areas, where gulls exploit unintentional and intentional feeding by people. They consume discarded food items, including popular snacks like bread, potato chips, and fast food scraps. Gulls have learned to use human cues to locate these food sources, preferentially pecking at items they observe a person handling or dropping. This learned association contributes to their success in urban environments, especially when natural prey availability is low.
Dietary Plasticity and Health Implications
The success of gulls is a direct result of their pronounced dietary plasticity, the biological ability to process and thrive on a wide variety of food types. This digestive flexibility allows them to switch between a marine-based diet of fish and invertebrates and a terrestrial diet of insects, seeds, and human refuse. This adaptation has allowed many gull populations to thrive in environments heavily altered by human development.
However, the shift away from natural marine prey carries distinct nutritional consequences for their long-term health. The anthropogenic diet, often composed of human junk food, is typically deficient in omega-3 long-chain polyunsaturated fatty acids (n3-LCPUFAs), which are essential for the structural development of cerebral tissues. Urban-nesting gulls that exploit rubbish have been found to have significantly lower cerebral n3-LCPUFAs compared to gulls foraging on natural marine food.
Human-made foods are often high in omega-6 polyunsaturated fatty acids (n6-PUFAs), which are pro-inflammatory compounds that can lead to adverse health effects when not balanced by omega-3s. This lower nutritional quality has been linked to negative reproductive outcomes, including reduced brood sizes and lower nestling body mass. Furthermore, scavenging at landfills and waste sites exposes gulls to harmful contaminants like heavy metals and flame-retardants, as well as non-digestible items such as plastic and broken glass.