Foraging and predation: Seagulls exhibit diverse foraging strategies, catching a variety of creatures as part of their diet. Here's a general breakdown:
- Marine Predators: Seagulls actively hunt and consume fish, small crustaceans, insects, and other aquatic organisms in the intertidal zone and near the water surface. They spot their prey from the air, swoop down, and catch them using their beaks.
- Scavenging Behavior: Seagulls readily consume dead or dying animals, including fish, marine mammals, birds, and even human-generated waste. They are often seen around fishing vessels, garbage dumps, or beaches where they can scavenge on discarded food.
- Predation on Other Birds: Some species of seagulls, like the Great Black-backed Gull, have been known to prey upon smaller seabirds, such as terns and gulls. They target nests and snatch up vulnerable eggs or chicks.
Secondary Consumers: In the marine food chain, seagulls fall under the category of secondary consumers. They feed upon smaller organisms, often primary consumers, which have consumed plant matter or microscopic zooplankton. For instance:
- Sandlance Predators: Seagulls commonly prey upon sandlances, small fish commonly found in near-shore waters. Sandlances, in turn, feed on copepods and other tiny creatures lower down in the food chain. By consuming sandlances, seagulls indirectly exert a top-down influence on their prey species.
- Herring Gulls and Jellyfish: Herring gulls, especially in the Pacific Northwest, have adapted to feed on jellyfish blooms in some areas. Jellyfish, although primarily carnivorous, consume planktonic organisms, like plankton and small fish. Thus, seagulls feeding on jellyfish influence the abundance of those gelatinous predators.
Competition for Food and Displacement: Seagulls often face competition from other seabirds, birds of prey, and marine mammals for shared food resources. In coastal habitats, they may compete with other predators like herons, egrets, and even larger gulls. Sometimes, seagulls can displace other species by being aggressive and dominating favored feeding grounds.
Examples of Top-Down Effects: In certain ecosystems, seagulls act as top predators and exert top-down regulatory effects. For instance:
- Black-headed Gulls and Terns: In parts of Europe, research suggests that Black-headed Gulls' predation on tern chicks can influence the breeding success of different tern species. This predator-prey interaction impacts tern populations and affects the overall ecological balance in coastal habitats.
- Predation on Marine Invertebrates: In certain regions, seagulls may feed intensely on specific marine invertebrate species, like mussel beds. By controlling invertebrate abundance, seagulls indirectly influence the availability of food resources for other marine organisms.
In summary, seagulls play important ecological roles as predators and scavengers in marine and coastal food webs. Their position within the food chain varies depending on their diet and habitat, and they can have significant impacts on the populations of their prey species, affecting the overall ecosystem dynamics.