Seabird
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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 and feeding niches 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. Other species such as the auks and cormorants tend to be more coastal. Some seabird species are marine for only part of the year, nesting inland in marshes and on lakes.
Seabirds live longer, breed later and have fewer young than other birds do, but they invest a great deal of time in those young that they do have. Most species nest in colonies, which can vary in size from a few dozen birds to many millions. They 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 feed on each other.
Seabirds and humans have a long history together, they have provided food to hunters, guided fishermen to fishing stocks and sailors to land. Many species are currently threatened by human activities, and conservation efforts are underway to preserve them.
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Classification of species as seabirds
There exists no one definition of which groups, families and species are seabirds, and most definitions are in some way aribitary. In the words of two seabird scientists "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" (Schreiber & Burger 2002). However by convention the penguins, tubenoses, all the Pelecaniformes except the darters, and some of the Charadriiformes (the skuas, gulls, terns, auks and skimmers) are all considered seabirds. The phalaropes are usually included as well, since although they are waders ("shorebirds" in North America), 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 included by some authors, although they are usually treated as waterbirds. 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. Many waders (or shorebirds) and herons are also highly marine, living on the sea's edge, but are also not treated as seabirds.
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.
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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
Adaptions to life at sea
Seabirds have made numerous adaptions to living on and feeding in the sea. Wing morphology has been shaped by the niche 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, whilst diving species have shorter wings. Species such as the Wandering Albatross, which forage over huge areas of sea, have almost lost the ability for powered flight and are dependent on a type of gliding called static soaring (where the wind deflected by waves provides lift). Seabirds also almost always have webbed feet, both to aid movement on the surface as well as used for diving in some species. The Procellariiformes are also unusual amongst birds in using a sense of smell (olfaction) in order to find widely distributed food in a vast ocean.
Salt glands are used by seabirds to deal with the salt they ingest by drinking and feeding (particluarly on crustaceans), and help them osmoregulate. The excretions from these glands (positioned in the head of the birds and emerging from the nasal cavity) are almost pure NaCl.
With the exception of the cormorants, all seabirds have waterprooof plumage in common with most other birds, however unlike terrestrial 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 allow water to soak their feathers as it allows them to swim without fighting the buoyancy that retaining air in the feathers causes, the cost of this is the need to dry out the feathers one they return to land.
The plumage of most seabirds is less colourful than that of landbirds, mostly restricted to variations of black, white or grey. A few species sport colourful plumes (like the tropicbirds or some penguins), but the majority of colour in seabirds comes from the bills and legs. The plumage of seabirds is thought in many cases to be for camoflage, both defensive (the colour of US Navy battleships is the same as that of Antarctic Prions!), and aggresive (the white underside possesed by many seabirds helps hide them from prey below them).
Seabird feeding
Seabirds evolved to exploit different food resources on 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 at sea feeding, 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, fish, squid or other prey items within reach of a dipped head. The distribution of prey on the sea's surface is often very patchy, some areas have large concentrations of food, while vast areas have none at all, so it is no coincidence that the species that exploit this resource are amongst the best fliers of any of the seabirds, including the greatest traveler of all, the albatross.
Surface feeding itself can be broken up into two different approaches, surface feeding while flying (for example as practiced by skimmers, frigate-birds and some storm-petrels) and surface feeding whilst swimming (examples of which are fulmars, gulls, many shearwaters and gadfly petrels. Surface feeders in flight include some of the most acrobatic of seabirds, either snatching morsels from the water (as do friagte-birds and some terns), or 'walking' on the water's surface, as do the storm-petrels. Another seabird family that does not land while feeding is the skimmer, which has a unique method; skimmers fly 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 relfects its unusual lifestlye, with the lower mandible uniquely being longer than the upper one.
Surface feeders that swim often have unique bills, adapted for their specific prey. Prions have special bills with filters called lamellae to filter out plankton from mouthfulls of water, and many albatross and petrels have hooked bills to snatch fast moving. Gulls have more generalised bills that reflect their more opportunistic lifestyle.
Pursuit diving
Pursuit diving exerts greater pressures (both evolutionary and physiological) on seabirds, but the reward is a greater area to feed in than is available to surface feeders. Propulsion underwater can be provided by wings (as used by penguins, auks, diving petrels, and shearwaters), or feet (as used by cormorants, grebes and divers (as well as several types of fish-eating ducks). Wing-propelled divers generally are faster than feet-propelled divers. In both cases the use of wings or feet for diving has limited its utility in other situations, divers and grebes walk with extreme difficulty (if at all), penguins cannot fly, and auks have sacrificed flight to a great extent for underwater diving. For example, the razorbill (and Atlantic auk) requires 64% more energy to fly as an equivalent petrel. Of all the wing-propelled pursuit divers the best in the air are the shearwaters, and it is no coincidence that they are the poorest divers. This is the dominant guild in polar and subpolar environments, as it is energetically inefficient in warmer waters. Because of their poor flying ability many pursuit divers are more confined in their foraging range than other guilds, especially during the breeding season when hungry chicks need regular feeding.
Plunge-diving
Gannets, boobies, tropicbirds, some terns and Brown Pelicans all engage in plunge-diving, taking fast moving prey by diving from flight into the water. This uses less enegry than dedicated pursuit divers, and they are able to use more spread out food resources, for example impoverished tropical seas. In general this is the most specialised method of hunting employed by seabirds, other non-specialists (such as gulls and skuas) may use it but do so with less skill and from lower hights. In Brown Pelicans the skills of plunge diving take several years to develop fully, once mature they can dive from 20 m above the water's surface, shifting the body before impact to avoid injury. Plunge divers are restricted in their hunting grounds to clear waters that afford a view of their prey from the air, and are the dominant guild in the tropics. Some plunge divers (as well as surface feeders) are dependent on predatory fish like tuna and dolphins to push shoaling fish up towards the surface.
Kleptoparasitism, scavenging and other seabirds
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Life-history
Seabirds' life-histories are dramatically different from those of land birds. In general they are K-selected, 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 a few exceptions, like 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.
Because of the greater investment in raising the young and because foraging for food may occur far from the nest site, in most species both parents participate in caring for the young and pairs are typically at least seasonally monogamous.
Seabird colonies
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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 Antarctica). 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.
Seabird colonies are highly variable. Individual nesting sites can be widely spaced, as in an albatross colony, or densely packed like a murre colony. In most seabird colonies several different species will nest on the same colony, often exibiting some niche separation. Seabirds can nest in trees (if any are available), on the ground (with or without nests), on cliffs, in burrows under the ground and in rocky crevises.
Many seabirds show remarkable site fidelity, returning to the same burrow, nest or site for many years, and they will defend that site from rivals with great vigour. This increases breeing success, provides an place for returning mates to reunite, and reduces the costs of prospecting for a new site. Young adults breeding for the first time usually return to their natal colony, and often nest very close to where they hatched.
Colonies are thought to provide protection to seabirds, which are often very clumsy on land, and are usually situated on islands where land mammals have difficulty getting to. Coloniality often arises in other types of bird which do not defend feeding territories (such as swifts, which have a very variable prey source), this may be a reason why it arises more frequently in seabirds. There are disadvantages to colonial life, particularly the spread of disease. There are other possible advantages, colonies may act as information centres, where seabirds returning to the sea to forage can find out where prey is by studying returning individuals of the same species.
Seabird migration
Other species also migrate shorter distances away from the breeding sites, their distribution at sea determined by the avaliablity of food. After fledging, juvenile birds often disperse further than adults to, and to different areas, and are commonly sighted far from a species' normal range. Some species, such as the auks, do not have a concerted migration effort, but drift southwards as the winter approaches. Other species, such as some of the storm-petrels, diving petrels and cormorants, never disperse at all, staying near the breeding colonies year round.
Away from the sea
Whilst the defentition of seabirds suggests that the birds in question spend their lives on the ocean, many seabird families have many species that spend some or even most of their lives inland away from the sea. Most strikingly, many species breed many tens, hundereds or even thousands of miles inland. Some of these species still return to the ocean to feed, for example the Snow Petrel, the nests of which have been found 300 miles inland on the Antarctic mainland, are unlikely to find anything to eat around their breeding sites. The Marbled Murrelet nests inland in old-growth forest, seeking ancient conifers with large branches to nest on. Other species, such as the California Gull nest and feed inland on lakes, and then move to the coasts in the winter. Some cormorants, pelican, gull and tern species have individuals that never visit the sea at all, spending their lives on lakes, rivers, swamps and, in the case of some gulls, cities and argicultural land. Some seabirds, principally those that nest in tundra like skuas and phalaropes, will migrate across land as well.
The more marine species, such as petrels, auks, and gannets, are more restricted in their habits, but are occasionally seen inland as vagrants. This most commonly happens to young inexperienced birds, but can happen in large numbers to exausted adults after a large storm, an event known as a wreck, where they are prized sightings for birders.
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).
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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. For example, the Maori of Stewart Island continue to harvest the chicks of the Sooty Shearwater as they have done so for centuries, using traditional methods (called kaitiakitanga) to manage the harvest, but now also work with the University of Otago in studying the populations.
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. Disturbance of breeding colonies by people is often a problem as well, visitors, even well meaning tourists, can flush a colony leaving chicks and eggs vunerable to predators.
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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. Oil spills are also a major threat to seabird species, as both a toxin and because the feathers of the birds become saturated by the oil, causing them to lose their waterproofing.
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.
Seabirds in culture
Gulls are one of the most commonly seen seabirds, given their use of human made habitats (such as cities and dumps), and their often fearless nature. They therefore also have made it into the popular consciousness, if only as the 'flying rats' berated in Finding Nemo. They have been used metaphorically, as in Jonathan Livingston Seagull, by Richard Bach, or to denote a closeness to the sea, such as their use in the Lord of the Rings, both in the insignia of Gondor, and therefore Numenor (used in the design of the film), and to call Legolas to, and across, the sea.
Seabird families
The following are the groups of birds normally classed as seabirds.
Sphenisciformes (Antarctic and southern waters; 16 species)
- Spheniscidae penguins
Procellariiformes (Tubenoses: pan-oceanic and pelagic; 93 species)
- Diomedeidae albatross
- Procellariidae fulmars, prions, shearwaters, gadfly and other petrels
- Pelacanoididae diving petrels
- Hydrobatidae storm-petrels
(see also petrel)
Pelecaniformes (Worldwide; 57 species)
- Pelecanidae pelicans
- Sulidae gannets and boobies
- Phalacrocoracidae cormorants
- Fregatidae frigatebirds
- Phaethontidae tropicbirds
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
- del Hoyo, Josep, Elliott, Andrew & Sargatal, Jordi (1992). Handbook of Birds of the World Vol 1. Barcelona:Lynx Edicions, ISBN 84-87334-10-5
- Gaston, Anthony J. & Jones, Ian L. (1998). The Auks Oxford:Oxford University Press, ISBN 0-19-854032-9
- Gaston, Anthony J. (2004). Seabirds: A Natural History New Haven:Yale University Press, ISBN 0-300-10406-5
- Löfgren, Lars (1984). Ocean Birds, Gothenburg:Nordbok, ISBN 0-394-53101-9
- Schreiber, Elizabeth A. & Burger, Joanne.(2001.) Biology of Marine Birds, Boca Raton:CRC Press, ISBN 0-8493-9882-7
External links
- Project Titi; a collaboration between the Maori of Stewart Island and the University of Otago to manage Sooty Shearwater harvests (http://www.otago.ac.nz/titi/default.html)
- BirdLife International; Save the Albatross Campaign (http://www.birdlife.org/action/campaigns/save_the_albatross/index.html)
- Marine Ornithology, the Journal of Seabird Science and Conservation (http://www.marineornithology.org/)