Seabirds are defined as species that rely on the marine environment for a significant portion of their annual food intake. This grouping includes a vast array of birds, from the towering albatrosses to the small storm-petrels, displaying a remarkable spectrum of sizes, behaviors, and physiology. Their collective reliance on the ocean has driven a high degree of specialization, resulting in diets and hunting techniques highly adaptive to the unpredictable nature of marine ecosystems.
The Core Menu: Primary Prey and Invertebrates
The foundational energy source for many seabirds consists of small, schooling pelagic fish, often referred to as forage fish. Species like anchovies, sardines, capelin, and sandeels are frequently targeted because they form dense aggregations and possess a high fat content, which provides the necessary energy for long flights and feeding offspring.
Beyond fish, marine invertebrates form a substantial part of the global seabird diet, particularly cephalopods and various crustaceans. Squid are a widely consumed prey item, and some species, such as albatrosses, may even scavenge squid that are too large to have been caught alive. The diet of many smaller seabirds is heavily dependent on zooplankton, such as krill, copepods, and amphipods.
Krill, which are small shrimp-like crustaceans, are particularly important in polar and sub-polar waters, acting as a major food web link between phytoplankton and top predators. Seabirds also engage in opportunistic feeding, especially around human activity. This involves scavenging on dead marine life or discarded fish waste from commercial fishing vessels.
Feeding Strategies and Methods of Acquisition
The methods seabirds use to acquire their prey are highly varied and depend on the bird’s physical adaptations and the location of the food source. One of the most common approaches is surface feeding, which can involve several distinct behaviors. Surface seizing is when a bird, such as a gull or petrel, lands on the water or dips its head while swimming to grasp prey near the surface.
Skimming is a specialized version used by birds like skimmers, which fly low with their lower mandible slicing the water, snapping shut when contact is made with a small fish or crustacean. Other surface feeders, like storm-petrels, use a pattering technique. They fly low and strike the water with their feet to create small disturbances, helping them maintain position while picking food from the surface film.
For deeper prey, seabirds utilize various forms of diving. Plunge diving involves birds like gannets and boobies diving from the air, hitting the water at high speed to capture fish just below the surface. Some species possess specialized air sacs that function as shock absorbers to protect them during these high-impact entries.
In contrast, pursuit divers actively chase their prey underwater, propelled by either their wings or their feet. Penguins and auks, including puffins and murres, use their small, stiff wings for underwater propulsion in wing-propelled diving. Cormorants use their large, webbed feet to paddle and maneuver through the water, employing a foot-propelled diving technique. The depth and duration of these dives are often limited by the bird’s body size and physiological capacity.
A less direct method of acquisition is kleptoparasitism, or food-stealing, employed by species like frigatebirds and skuas. These birds aggressively chase other seabirds until the pursued bird drops or regurgitates its freshly caught meal, which the pirate then snatches in mid-air. This behavior is a partial supplement to their hunting, often occurring when food resources are scarce.
Environmental Factors Shaping Seabird Diets
The geographical location of a seabird greatly influences its diet, reflecting the available marine resources in different biomes. Tropical seabirds, for example, frequently rely on prey like flying fish and squid, which are more prevalent in warmer waters. Conversely, seabirds in polar regions have diets heavily dominated by cold-water species like Arctic cod and the energy-rich krill.
Changes in oceanic conditions, such as fluctuations in sea surface temperature, can dramatically alter prey distribution and availability. Events like El Niño can cause warm water to suppress cold, nutrient-rich upwellings, forcing forage fish into deeper or different areas. This often leads to widespread dietary shifts or even starvation for seabirds, making them valuable indicators of marine ecosystem health.
The yearly cycle of breeding and migration requires significant dietary adjustments. During the breeding season, parents require highly nutritious and easily obtainable prey to feed their chicks, often switching to larger, fattier fish to meet the high energy demands of reproduction. Migration also dictates diet, as birds must exploit whatever suitable food sources are available along their extensive flight paths.
Long-term changes, such as the decline of ice-dependent species like Arctic cod due to warming trends, force species like thick-billed murres to switch to subarctic prey like capelin. This demonstrates how climate change directly reshapes seabird diets.