The Order Podicipediformes is limited to one extant family (Podicipedidae), which includes all the grebes. Approximately 22 species of grebes are recognized worldwide, classified in six genera. Grebes are small- to medium-sized, heavy-bodied birds with long necks and feet set far back on the body. They have an almost exclusive aquatic lifestyle and are limited in mobility when on land. Grebes inhabit freshwater and inland wetland habitats, although some species overwinter in salt water and may be migratory. Some species congregate in flocks of hundreds to thousands of birds and may migrate en masse, but most species are solitary or found in small groups. On average, male grebes are larger than females. Most species exhibit seasonally dichromatic plumage, and molt occurs on nonbreeding grounds. Grebes forage by diving for prey and are highly adapted divers and agile swimmers.

Members of the Order Gaviiformes, commonly known as loons or divers, are limited to one genus (Gavia) in one family (Gaviidae). There are five recognized species of loons worldwide. The term “loon” is used in North America and is synonymous with “diver” in the Old World. Loons are geographically limited to the Northern hemisphere (North America and Eurasia). Loons are long and heavy-bodied birds with webbed feet set far back on the body. Like the grebes, their lifestyle is almost exclusively aquatic, and they may have limited mobility on land. Breeding occurs near fresh water, but birds overwinter in marine environments and are migratory.

The Order Procellariformes includes the albatrosses, mollymawks, petrels, storm petrels, shearwaters, and diving petrels. Considerable size diversity exists within this order—from the small storm petrels that weigh 25 grams (g) as adults, to the albatrosses that exceed 10 kg and are among the largest birds capable of flight. The order is composed of four families: (1) Diomedeidea (albatrosses and mollymawks), (2) Procellaridae (petrels and shearwaters), (3) Hydrobatidae (storm petrels), and (4) Pelecanoidea (diving petrels). On the basis of recent molecular data, the Pelecanoidea, which consists of only one genus, should be classified as a subfamily within the Procellaridae.

Procellariformes are oceanic, pelagic species that spend very limited time on land except during nesting or breeding season. They are highly migratory, skilled long-distance fliers who may also be good swimmers. These birds rely on dynamic soaring and slope soaring to cover long distances in flight while conserving energy, and this is particularly true of the larger-bodied albatross species. Long distance migrations, sometimes for hundreds and thousands of miles, are essential for foraging on specialized diets in specific foraging grounds. Procellariformes are present throughout the world, but a distinct predominance of species exists in the Southern Hemisphere.

The Family Diomedeidae, which includes the albatrosses and mollymawks, comprises entirely pelagic oceanic species. Mollymawks are medium-sized albatrosses limited to southern oceans. The taxonomy of this family has undergone frequent revisions and has been the source of ongoing debate. As many as four genera have been proposed, with at least two being widely accepted. The number of distinct species ranges from 13 to 24, depending on taxonomic revisions. The greatest diversity of species occurs in the Southern Hemisphere. It has been hypothesized that the calmer winds found in the doldrums of the equator pose a geographic barrier to the northern dispersal of the albatross species. Many of the albatross species are threatened or at risk of extinction.

The albatrosses have the largest wingspans of any bird and may measure over 11 feet in some species, although the wing profile is only obvious in flight. The majority of flight is energy-efficient gliding, relying on wind speed. During calm wind conditions, albatrosses often choose to sit on the water. The family is extremely colonial, and birds nest in remote, isolated islands, with pairs that remain together for life. Birds feed by floating on the water surface and picking the prey around them.

The Family Procellaridae encompasses the petrels, shearwaters, and fulmars and includes a large number of small pelagic birds with drab plumage, which only come to shore during the breeding season. Most species migrate over long distances. Although found in all oceans, species diversity peaks in the Southern Hemisphere. The taxonomy of the Procellaridae is in a constant state of revision, with differences in opinion on numbers of distinct species (70–80) and genera (12–14). The Procellaridae have stout bodies with short tails, webbed feet, and monochromatic plumage that varies on the amount of black, gray-brown, and white coloration. Most species are colonial and nest in remote oceanic islands, primarily in underground burrows.

The Family Pelecanoidea, the diving petrels, consists of four species in a single genus (Pelecanoides) and is likely a subgroup of the Procellaridae and not a distinct family. Diving petrels are auk-like species, with geographic ranges limited to the southern oceans, but are some of the most numerous aquatic bird species. Most diving petrels are small, weighing between 100 and 200 g and have a characteristic black-and-white plumage. Diving petrels feed exclusively by underwater pursuit-diving of fish, squid, crustaceans, and other invertebrate prey. Some species of diving petrels may dive distances exceeding depths of 80 meters (m). Their small, stocky wings are also used for paddling and propulsion in the water.

The Family Hydrobatidae includes roughly 20 species of storm petrels, distributed in seven genera, although the taxonomy is being constantly revised and debated. Storm petrels are small, delicate birds with relatively large heads. In most species, the long legs dangle below the body, and the feet patter on the surface of the water when near the surface, giving the impression of the bird walking on the surface of the water. These pelagic species are found in all oceans. These birds may congregate in very large numbers. Storm-petrels feed primarily on phytoplankton and small invertebrates, with fish being consumed occasionally. Storm petrels are often predated by other birds and by introduced mammalian predators.

The members of the three orders included in this chapter face similar threats to their continued existence: anthropogenic habitat disturbance and modification, pollution, overharvesting, and (4) predation by introduced species. Some families (e.g., Diomedeidae) contain a large number of species at some risk of extinction. For primarily inland aquatic species (loons and grebes), the threats include the draining of wetland areas, human disturbance, pollution (specifically heavy metals and pesticides), and exposure to coastal oil spills during overwintering in marine environments. Marine seabirds are threatened by overharvesting for food, feathers, and oil; accidental bycatch during fishing operations; habitat disturbance of nesting grounds (including guano harvesting); pollution; and predation by introduced species (rats, cats, pigs, mongoose). Introduced predators may destroy or eat the eggs, the chicks, and the adults sitting on nesting sites, causing a significant impact on the populations in a relatively short timespan. Large-scale harvesting of fish by indiscriminate use of explosives, an illegal practice in many countries, may have a significant impact by affecting large groups of birds during feeding congregations. In addition, long-line fishing operations present a specific risk to many Procellariformes, as the birds get accidentally entangled in lines and die by drowning. Many species are prone to ingesting indigestible pieces of waste and garbage created by humans, specifically plastic, and this has been a threat to albatross species and their chicks. The epidemiology of infectious diseases has not been extensively studied in colonies, but the risk of virulent diseases spreading could be significant to the continued survival of some species, in particular those with limited nesting sites or isolated populations. The introduction and spread of foreign infectious diseases to established, naive colonies could have a significant effect at the population level.

Unique Anatomy

Members of all three orders have webbed feet. In loons and grebes, the feet are positioned caudally on the body and are the primary form of propulsion when swimming. The caudal positioning of the feet often limits the locomotor capabilities of these birds.

Grebes have specialized, lobated digits, which are specifically used for propulsive locomotion in the water. The tarsi are laterally compressed and the anterior digits (2, 3, and 4) have excess “lobes” of skin capable of contracting or expanding as the bird paddles when swimming. The nails are flat on the foot and do not extend like claws. As the foot is advanced in a cranial direction, the lobes are collapsed to minimize the profile and decrease the friction and drag against the water. The lobes are flared as the foot pushes back to form a paddle effect and propel the bird forward during a stroke. As highly adapted divers with caudally positioned feet, grebes have difficulty launching from the water for flight and often must use the rapid movements of their wings to propel themselves across the surface of the water before becoming airborne. Additional adaptation to swimming and diving in loons and grebes are a predominance of nonpneumatic bones and a decreased air sac system, which allow them better control of buoyancy.

Members of the Procellariformes have a characteristic tubelike extension of the nares extending on the dorsal aspect of the bill, well-developed salt glands, and a good sense of smell that is used for both prey detection and recognition of nesting sites. Some species use the large webs on their feet for maneuvering in flight as well as in swimming. All members have well-developed salt glands, which are located dorsal to the orbit and are used in salt homeostasis by excreting salt in drops over the bill. It is an adaptation that allows the ingestion of saltwater and saltwater prey without the need to drink fresh water. Salt metabolism is also regulated by renal excretion. The sense of smell is well developed in Procellariformes, and many species may detect the smell of certain oils in the water for locating food in the open sea. The sense of smell also serves a purpose in locating nesting sites or burrows, and this function may even be more significant than prey detection in some species of diving feeders, which may rely on few olfactory cues for prey location. Many species are well adapted for nocturnal vision, which may be crucial for the underground nesting species and for predator avoidance when returning to nesting sites at night. In some species, nocturnal vision likely is an adaptation for feeding at night.

Most Procellariform species are highly adapted for long distance flight and gliding, an energy-conserving strategy that allows the larger mass birds to travel long distances. The diving petrels are highly adapted for feeding underwater—including adaptations such as auk-like black-and-white cryptic coloration, a gular pouch, and short wings.