Seabird
Seabirds are birds that are adapted to life within the marine environment. While seabirds vary greatly in lifestyle, behaviour and physiology, they often exhibit striking convergent evolution, as the same environmental problems and feeding niches have resulted in similar adaptations. The first seabirds evolved in the Cretaceous period, while modern seabird families emerged in the Paleogene.
Seabirds generally live longer, breed later and have fewer young than many other birds, but they invest a great deal of time in their young. Most species nest in colonies, varying in size from a few dozen birds to millions. Many species are famous for undertaking long annual migrations, crossing the equator or circumnavigating the Earth in some cases. They feed both at the ocean's surface and below it, and even on each other. Seabirds can be highly pelagic, coastal, or in some cases spend a part of the year away from the sea entirely.
Seabirds and humans have a long history together: They have provided food to hunters, guided fishers to fishing stocks, and led sailors to land. Many species are currently threatened by human activities such as oil spills, fishing activity, offshore development, climate change and severe weather. Conservation efforts include the establishment of wildlife refuges and adjustments to fishing techniques.
Classification
There exists no single definition of which groups, families and species are seabirds, and most definitions are in some way arbitrary. Elizabeth Shreiber and Joanna Burger, two seabird scientists, said, "The one common characteristic that all seabirds share is that they feed in saltwater; but, as seems to be true with any statement in biology, some do not." However, by convention, all of the Sphenisciformes, all of the Phaethontiformes, all of the Procellariiformes, all of the Suliformes except the darters, one family of the Pelecaniformes, and some of the Charadriiformes are classified as seabirds. The phalaropes are usually included as well, since although they are waders, two of the three species are oceanic for nine months of the year, crossing the equator to feed pelagically.Loons and grebes, which nest on lakes but winter at sea, are usually categorized as water birds, not seabirds. Although there are a number of sea ducks in the family Anatidae that are truly marine in the winter, by convention they are usually excluded from the seabird grouping. Many herons and waders, such as crab-plovers, are also highly marine, living on the sea's edge, but are also not treated as seabirds. Fish-eating birds of prey, such as sea eagles and ospreys, are also typically excluded, however tied to marine environments they may be. Some birds, such as darters, are primarily found in freshwater habitats, but may occasionally venture into marine or coastal areas as well; such birds are generally not considered to be seabirds.
German ornithologist Gerald Mayr defined the "core waterbird" clade Aequornithes in 2010. This lineage gives rise to the Procellariiformes, Sphenisciformes, Suliformes, Pelecaniformes, Ciconiiformes, and Gaviiformes. The tropicbirds are part of the Eurypygimorphae lineage, which is sister to the Aequornithes; this clade also includes the non-seabird Eurypygiformes. The Charadriiformes are more distantly related to the other seabirds, being more closely related to the non-seabird Gruiformes and Opisthocomiformes in the clade Gruae.
Evolution and fossil record
Seabirds, by virtue of living in a geologically depositional environment, are well-represented in the fossil record. They are first known to occur in the Cretaceous period, the earliest being the Hesperornithes. These were flightless seabirds that could dive in a fashion similar to grebes and loons, but had beaks filled with sharp teeth. Other Cretaceous seabirds included the gull-like Ichthyornithes. Flying Cretaceous seabirds do not exceed wingspans of two meters; piscivorous pterosaurs occupied seagoing niches above this size.While Hesperornis is not thought to have left descendants, the earliest modern seabirds also occurred in the Cretaceous, with a species called Tytthostonyx glauconiticus, which has features suggestive of Procellariiformes and Fregatidae. As a clade, the Aequornithes either became seabirds in a single transition in the Cretaceous or some lineages such as pelicans and frigatebirds adapted to sea living independently from freshwater-dwelling ancestors. In the Paleogene both pterosaurs and marine reptiles became extinct, allowing seabirds to expand ecologically. These post-extinction seas were dominated by early Procellariidae, giant penguins and two extinct families, the Pelagornithidae and the Plotopteridae. Modern genera began their wide radiation in the Miocene, although the genus Puffinus might date back to the Oligocene. Within the Charadriiformes, the gulls and allies became seabirds in the late Eocene, and then waders in the middle Miocene. The highest diversity of seabirds apparently existed during the Late Miocene and the Pliocene. At the end of the latter, the oceanic food web had undergone a period of upheaval due to extinction of considerable numbers of marine species; subsequently, the spread of marine mammals seems to have prevented seabirds from reaching their erstwhile diversity.
Characteristics
Adaptations to life at sea
Seabirds have made numerous adaptations to living on and feeding in the sea. Wing morphology has been shaped by the niche in which an individual species or family has evolved, so that looking at a wing's shape and loading can tell a scientist about its life feeding behaviour. Longer wings and low wing loading are typical of more pelagic species, while diving species have shorter wings and high wind loading. Species such as the wandering albatross, which forage over huge areas of sea, have a reduced capacity for powered flight and are dependent on a type of gliding called dynamic soaring as well as slope soaring. Seabirds also almost always have webbed feet, to aid movement on the surface as well as assisting diving in some species. The Procellariiformes are unusual among birds in having a strong sense of smell, which is used to find widely distributed food in a vast ocean, and help distinguish familiar nest odours from unfamiliar ones.File:Phalacrocorax-auritus-007.jpg|thumb|left|Cormorants, like this double-crested cormorant, have plumage that is partly wettable. This functional adaptation balances the competing requirement for thermoregulation against that of the need to reduce buoyancy.
Salt glands are used by seabirds to deal with the salt they ingest by drinking and feeding, and to help them osmoregulate. The excretions from these glands are almost pure sodium chloride.
With the exception of the cormorants and some terns, and in common with most other birds, all seabirds have waterproof plumage. However, compared to land birds, they have far more feathers protecting their bodies. This dense plumage is better able to protect the bird from getting wet, and cold is kept out by a dense layer of down feathers. The cormorants possess a layer of unique feathers that retain a smaller layer of air but otherwise soak up water. This allows them to swim without fighting the buoyancy that retaining air in the feathers causes, yet retain enough air to prevent the bird losing excessive heat through contact with water.
The plumage of most seabirds is less colourful than that of land birds, restricted in the main to variations of black, white or grey. A few species sport colourful plumes, but most of the colour in seabirds appears in the bills and legs. The plumage of seabirds is thought in many cases to be for camouflage, both defensive and aggressive. The usually black wing tips help prevent wear, as they contain melanins that help the feathers resist abrasion. Seabirds can also display fluorescence, particularly on their bills, likely to indicate their fitness to potential mates.
Diet and feeding
Seabirds evolved to exploit different food resources in the world's seas and oceans, and to a great extent, their physiology and behaviour have been shaped by their diet. These evolutionary forces have often caused species in different families and even orders to evolve similar strategies and adaptations to the same problems, leading to remarkable convergent evolution, such as that between auks and penguins. There are four basic feeding strategies, or ecological guilds, for feeding at sea: surface feeding, pursuit diving, plunge diving, and predation of higher vertebrates; within these guilds, there are multiple variations on the theme.Surface feeding
Many seabirds feed on the ocean's surface, as the action of marine currents often concentrates food such as krill, forage fish, squid, or other prey items within reach of a dipped head.Surface feeding itself can be broken up into two different approaches, surface feeding while flying, and surface feeding while swimming. Surface feeders in flight include some of the most acrobatic of seabirds, which either snatch morsels from the water, or "walk", pattering and hovering on the water's surface, as some of the storm petrels do. Many of these do not ever land in the water, and some, such as the frigatebirds, have difficulty getting airborne again should they do so. Another seabird family that does not land while feeding is the skimmer, which has a unique fishing method: flying along the surface with the lower mandible in the water—this shuts automatically when the bill touches something in the water. The skimmer's bill reflects its unusual lifestyle, with the lower mandible uniquely being longer than the upper one.
Surface feeders that swim often have unique bills as well, adapted for their specific prey. Prions have special bills with filters called lamellae to filter out plankton from mouthfuls of water, and many albatrosses and petrels have hooked bills to snatch fast-moving prey. On the other hand, most gulls are versatile and opportunistic feeders who will eat a wide variety of prey, both at sea and on land.