Birds that have adapted to life at sea spend their lives in the air above the surface, in the upper layers of the open ocean, or along shorelines. Shorebirds (birds that live and feed on the coastlines) rarely range far from land. These include wading birds, such as herons and cranes, and gulls. Seabirds, on the other hand, are pelagic, often remaining at sea for months on end and retuning to land only to breed. Unlike land birds, many pelagic sea birds breed in large colonies on islands and cliffs, deserting them when the breeding season ends.
There is no such thing as a typical sea bird, although pelagic birds share many adaptations for life at sea. These include webbed feet, highly waterproof plumage, and glands that get rid of excess salt. Most land birds have hollow, air-filled bones that help to save weight, but in diving species, such as penguins, the bones are denser and the air space reduced. Some plunge-divers, including gannets and pelicans, have air sacs under their skin. These cushion the impact as they hit the water and help them to bob back to the surface with their prey. Compared to these marine species, shorebirds show few specific adaptations for life in or near salt water but, like all birds, they have bills specialized for dealing with different kinds of food.
B. Adaptations of Seabirds:
Seabirds are well adapted to life in a marine environment, and they have several physical adaptations that give them an advantage for life on the high seas. Knowing these adaptations can help birders instantly recognize if they are looking at a seabird.
Plumage Coloration: Most seabirds have drab plumage that is dark above and light below. This type of coloration, called countershading, offers camouflage from aerial predators and hides the bird from potential prey beneath them.
Feathers: Seabirds have more feathers relative to their body size than other birds, which gives them superior waterproofing and insulation. Some seabird species have even more specialized feathers to help with buoyancy or additional insulation, such as penguins' dense feathers that protect them from Antarctic temperatures.
Feet: Most seabirds have flexible webbed feet that help them be powerful swimmers or help propel them across the water to gain speed for takeoff. Some species have strong claws on their feet to help with fishing as well.
Wing Shape: Seabirds' wings are specially shaped for their unique flight needs. Longer, more tapered wings allow seabirds to soar for hours with very little effort, allowing them to remain aloft far from land. In some species, shorter wings give the birds better control and agility for flight right at the surface of the water.
Salt Glands: Many seabirds have specialized salt glands that extract salt from the birds' food and water, allowing them to eat and drink without dehydration from too much salt. Excess salt is then excreted, typically near the birds' nostrils.
Head Structure: Seabirds that hunt deeper in the water through plunge diving, such as gannets, have specialized adaptations in their head structure to withstand high speed impacts with the water such as strong, tapered bills, air sacs and thicker bones.
C. Seabird Behavior
Because of their unique environment, seabirds act uniquely. Understanding the unique behaviors of seabirds can help people appreciate these marine birds.
Diet: Because of where they live, seabirds subsist on a diet of primarily fish, squid and crustaceans. Many seabirds will mob fishing boats in search of easy food from discarded fish and offal, and seabirds have also been known to forage through trash dumped at sea.
Flight: Most seabirds are powerful fliers with the ability to soar without effort on their long wings. The gliding, soaring flight of seabirds can be easily recognized, though smaller types of seabirds do have more energetic and erratic flight patterns.
Breeding: Unlike the solitary breeding preferences of many bird species, seabirds are colonial nesters and a breeding colony may grow to thousands of birds. Most seabirds nurture their young for much longer than typical land birds, and it is not unusual for a seabird fledgling to remain with its parents for several months at sea. Because of this long parental care period, most seabirds raise only one broodper year, and that brood is often just a single egg.
Lifespan: While a typical songbird may live just a handful of years and a bird older than 10 is unusual, seabirds have much longer lifespan. Many seabirds do not fully mature until they are several years old, and a lifespan of 20-40 years is not uncommon.
D. Bill Adaptations
Apart form waterfowl, most birds of the sea and shore are carnivores, with bills that are adapted for different kinds of animal prey. A pelican’s bill and pouch work like a scoop, while an albatross’s hooked bill can grip slippery prey, such as jellyfish. Sea eagles catch their prey with their talons, but then use their bills to tear it into pieces. Curlews have long bills that can probe for animals buried in mud.
Birds live throughout the world’s oceans and shorelines, from the equator to the poles. Less than 200 species are truly pelagic. Although they feed on sea animals, their true habitat is the air. Food is widely scattered in the open oceans, which is why the majority of sea birds live closer to land. Most diving sea birds feed in the shallow waters over continental shelves, while rocky coasts and mudflats are key habitats for waders and gulls. Estuaries are important habitats for coastal birds. Their muddy silt often harbors numerous worms and mollusks, accessible at low tide. In the tropics, mangrove swamps attract birds for the same reason; they also have the added bonus of trees, in which birds nest and roost.
F. Feeding methods:
Marine birds have evolved several ways of hunting their food.
Plunge divers – birds such as brown pelicans, which slam into shoals of fish from heights of up to 100 ft. Diving sea birds also include many that operate from the surface, such as cormorants and penguins.
Many oceanic birds, such as albatrosses and petrels, hunt on the wing, snatching animals or scraps from the surface. Some birds, such as frigate birds, harass other birds into releasing their catch.
Coastal birds often probe for food in the shallows or along the tide line.
Skimmers slice through the water, holding their lower bill underwater while in flight, but this technique works only if the surface is flat and calm.
G. Dispersal and Migration
Marine birds can range over a huge distance in their lifetime. Some disperse over wide areas of ocean, returning to isolated colonies to breed. Many other birds migrate between distinct summer and winter ranges.