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Seabirds are birds that spend much of their lives, outside the breeding season at least, at sea. Whilst the seabirds vary greatly in lifestyle, behaviour and physiology, they often exibit striking convergent evolution as the same environmental problems have resulted in similar adaptions.
Some seabird species, such as the albatrosses and petrels are truly pelagic, breeding on sea cliffs and small islands, and wintering on the open ocean. They are totally dependent on the sea for food. Many of these deep ocean species can barely walk on land.
Other species such as the auks are equally dependent on cliff-ledge breeding sites and marine prey, but tend to be more coastal once they have migrated from their breeding stacks.
Some of the less oceanic species, like grebes and divers, breed in freshwater environments but move to the coasts in winter. A few, such as the Great Crested Grebe and the Anhinga, will also winter on freshwater lakes where they remain unfrozen.
Unlike marine mammals, all seabirds, like this Waved Albatross, need to return to land to breed.
Evolution and fossil record of seabirds
Seabirds, by virtue of living in a geologically depositional environment (that is, in the sea where sediments are readily laid down) are well represented in the fossil record. They are first known to occur in the Cretaceous era, the earliest being the Hesperornithiformes, like Hesperornis regalis, a flightless loon-like seabird that dove in a similar fashion to loons and cormorants (using its feet to move underwater) but had a beak filled with sharp teeth. The Creatceous seabird Hesperornis While Hesperornis is not thought to have left descendants, the earliest extant seabirds also occurred in the Cretaceous, with a species called Tytthostonyx glauconiticus, which has been placed in the Procellariiformes. In the Paleogene the seas were dominated by early Procellariidae, giant penguins and two extinct families, the Pelagornithidae and the Plotopteridae (a group of large seabirds that looked like the penguins). Modern genera began their wide radiation in the Miocene although the genus Puffinus (which includes today's Manx Shearwater and Sooty Shearwater) dates back to the Oligocene.
Characteristics of seabirds
Life-history Seabirds' life-histories are dramatically different from those of land birds. In general they live much longer (anywhere between 20 and 60 years), they delay breeding for longer (for up to 10 years), and invest more effort into fewer young. Most species will only have one clutch a year, unless they lose the first (with the exception of the Cassin's Auklet), and many species (like the tubenoses and sulids), only one egg a year.
There is also a long period of care for the young, extending for as long as six months, among the longest for birds. For example, once Common Guillemot chicks fledge they remain with the male parent for several months at sea. This life-history stratergy has probably evolved both in response to the challanges of living at sea and the relative lack of predation compared to that of land living birds.
Seabird colonies 95% of seabirds (not including the grebes and loons) are colonial, and seabird colonies are amongst the largest in the world, and provide one of Earth's great wildlife spectacles. Colonies of over a million birds have been recorded, both in the tropics (such as Kiritimati in the Pacific) and in the polar latitudes (as found in Antartica). Seabird colonies occur exclusively for the purpose of breeding, non-breeding birds will only collect together outside the breeding season in areas where prey species are densely aggregated.
Seabirds and humans
Seabirds and fisheries Seabirds have had a long association with both fisheries and sailors. Both fisheries and seabirds have drawn benefits and disadvantages from the long relationship.
Fishermen have long used seabirds as indicators of both fish shoals, underwater banks that might indicate fish stocks, and of potential land. In fact the known association of seabirds with land was instrumental in allowing the Polynesians to locate tiny landmasses in the Pacific. Seabirds also provided food for fishermen away from home, as well as bait. Famously tethered cormorants have been used to catch fish directly. Indirectly fisheries have also benefited from guano from colonies of seabirds acting as fertilizer for the surrounding seas.
Negative effects on fisheries are mostly restricted to raiding by birds on aquaculture, although longlining fisheries also have to deal with bait-stealing. There have been claims of prey-depletion by seabirds of fishery stocks, and while there is some small evidence of this, the effects of seabirds are considered smaller than that of marine mammals and predatory fish (like tuna).
Some species and families of seabirds have benefited from fisheries, particularly from discarded fish and offal. These discards comprise 30% of the food of seabirds in the North Sea, for example, and comprise up to 70% of the total food of some species. This can have other impacts, for example the spread of the Northern Fulmar through the British Isles is attributed in part to the availability of discards. Discards generally benefit surface feeders, such as gannets and petrels to the detriment of pursuit divers like penguins.
Fisheries also have negative effects on seabirds, and these effects, particularly on the long lived and slow breeding albatross are a source of increasing concern to conservationists. The by-catch of seabirds entangled in nets or hooked on fishing lines has had a big impact on seabird numbers, for example an estimated 44,000 albatross are hooked each year on tuna lines set out by Japanese fleets. Overall many hundreds of thousands or possibly millions of birds are trapped and killed each year, a source of concern for some of the rarest species (for example, only 1,000 Short-tailed Albatross are known to still exist). Seabirds also suffer when stocks of fish are overfished.
Exploitation of seabirds The hunting of seabirds, and the collecting of seabird eggs, have contributed to the declines of many species of seabirds, as well as the extinction of one species, the Great Auk. Seabirds have been hunted for food by many coastal people over time, and have become locally extinct in many places. In particular at least 20 species out of 29 no longer breed on Easter Island. In the 19th century the hunting of seabirds for fat deposits and feathers for the millinery trade reached industrial levels. In the Falkland Islands hundreds of thousands of penguins were harvested for their oil each year. Seabird eggs have also long been an important source of food for sailors undertaking long sea voyages, as well as being taken when cities grow in areas near a colony. Eggers from San Francisco took almost half a millions eggs a year from the Farallon Islands in the mid 19th century, a period in the island's history from which the seabird species are still recovering.
Both hunting and egging continue today, although not at the level that occurred in the past, and generally in a more controlled level.
Other threats Other human caused factors have lead to declines and even extinctions in seabird populations, colonies and species. Of these, perhaps the most serious are introduced species. Seabirds, breeding overall on small isolated islands, have lost many predator defense behaviour. Feral cats are capable of taking out seabirds as large as albatross, and many introduced rodents, such as the Pacific rat can take eggs hidden in burrows. Introduced goats, cattle, rabbits and other herbivores can also lead to problems, particularly when species need vegetation to protect or shade their young.
The build up of toxins and pollutants in seabirds is also a concern. Seabirds, being apex predators, suffered from the ravages of DDT until they were banned, and concern continues with other pollutants, for example Forster's Terns in San Francisco were found to have high levels of polybrominated diphenyl ethers (PBDEs), used as fire retardants.
Seabird conservation The threats faced by seabirds have not gone unnoticed by scientists or the conservation movement. As early as 1903 Theodore Roosevelt was convinced of the need to declare Pelican Island in Florida a National Wildlife Refuge to protect the bird colonies (including the nesting Brown Pelicans), a few years later in 1909 he protected the Farallon Islands. Today many important seabird colonies are given some measure of protection, from Heron Island in Australia to Triangle Island in British Columbia.
The field of island restoration, developed initially by New Zealand, is removing exotic invanders from increasingly large islands. Feral cats have been removed from Wake Island, rabbits from Laysan, and rats from Campbell Island. The removal of these introduced species has lead to increases in surviving species and even the return of expirated ones.
The plight of albatross and other large seabirds, as well as other marine creatures, being taken as by-catch by longline fisheries, has been taken up by a large number of NGOs (including BirdLife International and the RSPB). This lead to The Agreement for the Conservation of Albatrosses and Petrels signed as part of the Convention on Migratory Species, a legally binding treaty designed to protect these threatened species (it has currently been signed by 6 countries, Argentina, Australia, Ecuador, New Zealand, Spain and South Africa.
Seabird families There exists no one definition of which groups are seabirds, however conventionally the penguins, tubenoses, Pelecaniformes, skuas, gulls, terns, skimmers and skimmers are in this group. Loons and grebes are also included by most scientists. Sometimes the phalaropes are included in the seabirds, since although they are waders ("shorebirds" in North America), two of the three species are oceanic outside the breeding season. Although there are a number of sea ducks in the family Anatidae which are truly marine, by convention they are usually excluded from the seabird grouping.
The following are the groups of birds normally classed as seabirds.
Sphenisciformes (Antarctic and southern waters; 16 species)
Gaviiformes (North America, Eurasia; 4 species) Podicipediformes (Worldwide; 20 species) Procellariiformes (Tubenoses: pan-oceanic and pelagic; 93 species) (see also petrel)
Pelecaniformes (Worldwide; 57 species)
Charadriiformes (Worldwide; 305 species, but only the families listed are classed as seabirds.) For an alternative taxonomy of these groups, see also Sibley-Ahlquist taxonomy.
See also list of birds.
References Schreiber, E.A. & Burger, J. Biology of Marine Birds, 2001, ISBN 0-8493-9882-7 | 
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