Distribution and movements
The Procellariiformes have a cosmopolitan distribution across the world's oceans and seas, although at the levels of family and genus there are some clear patterns. Antarctic petrels, Thalassoica antarctica, have to fly over 100 mi (160 km) to get to the ocean from their breeding colonies in Antarctica, and northern fulmars breed on the northeastern tip of Greenland, the furthest north piece of land. The most cosmopolitan family is the Procellariidae, which are found in tropical, temperate and polar zones of both the Northern and the Southern Hemispheres, though the majority do not breed in the tropics, and half the species are restricted to southern temperate and polar regions. The gadfly petrels, Pterodroma, have a generally tropical and temperate distribution, whereas the fulmarine petrels are mostly polar with some temperate species. The majority of the fulmarine petrels, along with the prions, are confined to the Southern Hemisphere.
The storm petrels are almost as widespread as the procellariids, and fall into two distinct subfamilies; the Oceanitinae have a mostly Southern Hemisphere distribution and the Hydrobatinae are found mostly in the Northern Hemisphere. Amongst the albatrosses the majority of the family is restricted to the Southern Hemisphere, feeding and nesting in cool temperate areas, although one genus, Phoebastria, ranges across the north Pacific. The family is absent from the north Atlantic, although fossil records indicate they bred there once. Finally the diving-petrels are restricted to the Southern Hemisphere.
The various species within the order have a variety of migration strategies. Some species undertake regular trans-equatorial migrations, such as the sooty shearwater which annually migrates from its breeding grounds in New Zealand and Chile to the North Pacific off Japan, Alaska and California, an annual round trip of 64,000 km (40,000 mi), the longest measured annual migration of any bird. A number of other petrel species undertake trans-equatorial migrations, including the Wilson's storm petrel and the Providence petrel, but no albatrosses cross the equator, as they rely on wind assisted flight. There are other long-distance migrants within the order; Swinhoe's storm petrels breed in the western Pacific and migrates to the western Indian Ocean, and Bonin petrels nesting in Hawaii migrate to the coast of Japan during the non-breeding season.
Many species in the order travel long distances over open water but return to the same nest site each year, raising the question of how they navigate so accurately. The Welsh naturalist Ronald Lockley carried out early research into animal navigation with the Manx shearwaters that nested on the island of Skokholm. In release experiments, a Manx shearwater flew from Boston to Skokholm, a distance of 3,000 miles (4,800 kilometres) in 121⁄2 days.
Lockley showed that when released "under a clear sky" with sun or stars visible, the shearwaters oriented themselves and then "flew off in a direct line for Skokholm", making the journey so rapidly that they must have flown almost in a straight line. But if the sky was overcast at the time of release, the shearwaters flew around in circles "as if lost" and returned slowly or not at all, implying that they navigated using astronomical cues.
Morphology and flight
Procellariiformes range in size from the very large wandering albatross, at 11 kg (24 lb) and a 3.6-metre (12-foot) wingspan, to tiny birds like the least storm petrel, at 20 g (0.71 oz) with a 32-centimetre (13-inch) wingspan, and the smallest of the prions, the fairy prion, with a wingspan of 23 to 28 cm (9.1 to 11.0 in). They have their nostrils enclosed in one or two tubes on their straight, deeply grooved bills with hooked tips. The beaks are made up of several plates. Their wings are long and narrow; the feet are webbed, and the hind toe is undeveloped or non-existent; their adult plumage is predominantly black, white, and grey.
The order has a few unifying characteristics, starting with their tubular nasal passage which is used for olfaction. This ability to smell helps to locate patchily distributed prey at sea and may also help locate their nests within nesting colonies. The structure of the bill, which contains seven to nine distinct horny plates, is another unifying feature, although there are differences within the order. Petrels have a plate called the maxillary unguis that forms a hook on the maxilla. The smaller members of the order have a comb-like mandible, made by the tomial plate, for plankton feeding. Most members of the order are unable to walk well on land, and many species visit their remote breeding islands only at night. The exceptions are the huge albatrosses, several of the gadfly petrels and shearwaters and the fulmar-petrels. The latter can disable even large predatory birds with their obnoxious stomach oil, which they can project some distance. This stomach oil, stored in the proventriculus, is a digestive residue created in the foregut of all tubenoses except the diving petrels, and is used mainly for storage of energy-rich food during their long flights. The oil is also fed to their young, as well as being used for defence.
Procellariiformes drink seawater, so they have to excrete excess salt. All birds have an enlarged nasal gland at the base of the bill, above the eyes, and in the Procellariiformes the gland is active. In general terms, the salt gland removes salt from the system and forms a 5 percent saline solution that drips out of the nostrils, or is forcibly ejected in some petrels. The processes behind this involve high levels of sodium ion reabsorption into the blood plasma within the kidneys, and secretion of sodium chloride via the salt glands using less water than was absorbed, which essentially generates salt-free water for other physiological uses. This high efficiency of sodium ion absorption is attributed to mammalian-type nephrons.
Most albatrosses and procellariids use two techniques to minimise exertion while flying, namely, dynamic soaring and slope soaring. The albatrosses and giant petrels share a morphological adaptation to aid in flight, a sheet of tendon which locks the wing when fully extended, allowing the wing to be kept up and out without any muscle effort. Amongst the Oceanitinae storm-petrels there are two unique flight patterns, one being surface pattering. In this they move across the water surface holding and moving their feet on the water's surface while holding steady above the water, and remaining stationary by hovering with rapid fluttering or by using the wind to anchor themselves in place. A similar flight method is thought to have been used by the extinct petrel family Diomedeoididae. The white-faced storm petrel possesses a unique variation on pattering: holding its wings motionless and at an angle into the wind, it pushes itself off the water's surface in a succession of bounding jumps.
Diet and feeding
The Procellariiformes are for the most part exclusively marine foragers; the only exception to this rule are the two species of giant petrel, which regularly feed on carrion or other seabirds while on land. While some other species of fulmarine and Procellaria petrels also take carrion, the diet of most species of albatrosses and petrels is dominated by fish, squid, krill and other marine zooplankton. The importance of these food sources varies from species to species and family to family. For example, of the two albatross species found in Hawaii, the takes mostly fish, while the Laysan feeds mainly on squid. The albatrosses in general feed on fish, squid and krill. Among the procellariids, the prions concentrate on small crustacea, the fulmarine petrels take fish and krill but little squid, while the Procellaria petrels consume mainly squid. The storm petrels take small droplets of oil from the surface of the water, as well as small crustaceans and fish.
Petrels obtain food by snatching prey while swimming on the surface, snatching prey from the wing or diving down under the water to pursue prey. Dipping down from flight is most commonly used by the gadfly petrels and the storm petrels. There have been records of wedge-tailed shearwaters snatching flying fish from the air, but as a rule this technique is rare. Some diving birds may aid diving by beginning with a plunge from the air, but for the most part petrels are active divers and use their wings to move around under the water. The depths achieved by various species were determined in the 1990s and came as a surprise to scientists; short-tailed shearwaters have been recorded diving to 70 m (230 ft) and the Light-mantled sooty albatross to 12 m (39 ft).
All Procellariiformes are colonial, predominantly breeding on offshore or oceanic islands. The few species that nest on continents do so in inhospitable environments such as dry deserts or on Antarctica. These colonies can vary from the widely spaced colonies of the giant petrels to the dense 3.6 million-strong colonies of Leach's storm petrels. For almost all species the need to breed is the only reason that Procellariiformes return to land at all. Some of the larger petrels have to nest on windswept locations as they require wind to take off and forage for food. Within the colonies, pairs defend usually small territories (the giant petrels and some albatrosses can have very large territories) which is the small area around either the nest or a burrow. Competition between pairs can be intense, as is competition between species, particularly for burrows. Larger species of petrels will even kill the chicks and even adults of smaller species in disputes over burrows. Burrows and natural crevices are most commonly used by the smaller species; all the storm petrels and diving petrels are cavity nesters, as are many of the procellariids. The fulmarine petrels and some tropical gadfly petrels and shearwaters are surface nesters, as are all the albatrosses.
Procellariiformes show high levels of philopatry, both site fidelity and natal philopatry. Natal philopatry is the tendency of an individual bird to return to its natal colony to breed, often many years after leaving the colony as a chick. This tendency has been shown through ringing studies and mitochondrial DNA studies. Birds ringed as chicks have been recaptured close to their original nests, sometimes extremely close; in the Laysan albatross the average distance between hatching site and the site where a bird established its own territory was 22 m (72 ft), and a study of Cory's shearwaters nesting near Corsica found that nine out of 61 male chicks that returned to breed at their natal colony actually bred in the burrow they were raised in. Mitochondrial DNA provides evidence of restricted gene flow between different colonies, strongly suggesting philopatry.
The other type of philopatry exhibited is site fidelity, where pairs of birds return to the same nesting site for a number of years. Among the most extreme examples known of this tendency was the fidelity of a ringed northern fulmar that returned to the same nest site for 25 years. The average number of birds returning to the same nest sites is high in all species studied, with around 91 percent for Bulwer's petrels, and 85 percent of males and 76 percent of females for Cory's shearwaters (after a successful breeding attempt).
Pair bonds and life history
Procellariiformes are monogamous breeders and form long-term pair bonds. These pair bonds take several years to develop in some species, particularly with the albatrosses. Once formed, they last for many breeding seasons, in some cases for the life of the pair. Petrel courtship can be elaborate. It reaches its extreme with the albatrosses, where pairs spend many years perfecting and elaborating mating dances. These dances are composed of synchronised performances of various actions such as preening, pointing, calling, bill clacking, staring, and combinations of such behaviours (like the sky-call). Each particular pair will develop their own individual version of the dance. The breeding behaviour of other Procellariiformes is less elaborate, although similar bonding behaviours are involved, particularly for surface-nesting species. These can involve synchronised flights, mutual preening and calling. Calls are important for helping birds locate potential mates and distinguishing between species, and may also help individuals assess the quality of potential mates. After pairs have been formed, calls serve to help them reunite; the ability of individuals to recognise their own mate has been demonstrated in several species.
Procellariiformes are K-selected, being long-lived and caring extensively for their few offspring. Breeding is delayed for several years after fledging, sometimes for as long as ten years in the largest species. Once they begin breeding, they make only a single breeding attempt per nesting season; even if the egg is lost early in the season, they seldom re-lay. Much effort is placed into laying a single (proportionally) large egg and raising a single chick. Procellariiformes are long-lived: the longest living albatross known survived for 51 years, but was probably older, and even the tiny storm-petrels are known to have survived for 30 years.
Nesting and chick rearing
The majority of Procellariiformes nest once a year and do so seasonally. Some tropical shearwaters, like the Christmas shearwater, are able to nest on cycles slightly shorter than a year, and the large great albatrosses (genus Diomedea) nest in alternate years (if successful). Most temperate and polar species nest over the spring-summer, although some albatrosses and procellariids nest over the winter. In the tropics, some species can be found breeding throughout the year, but most nest in discreet periods. Procellariiformes return to nesting colonies as much as several months before laying, and attend their nest sites regularly before copulation. Prior to laying, females embark on a lengthy pre-laying exodus to build up energy reserves in order to lay the exceptionally large egg. In the stormy petrel, a very small procellariiform, the egg can be 29 percent of the body weight of the female, while in the grey-faced petrel, the female may spend as much as 80 days feeding out at sea after courtship before laying the egg.
When the female returns and lays, incubation is shared between the sexes, with the male taking the first incubation stint and the female returning to sea. The duration of individual stints varies from just a few days to as much as several weeks, during which the incubating bird can lose a considerable amount of weight. The incubation period varies from species to species, around 40 days for the smallest storm-petrels but longer for the largest species; for albatrosses it can span 70 to 80 days, which is the longest incubation period of any bird.
A Laysan albatross
feeds its chick. The parent pumps food from a modified foregut, the proventriculus, and the chick catches the meal in its lower mandible.
Upon hatching, the chicks are semi-precocial, having open eyes, a dense covering of white or grey down feathers, and the ability to move around the nesting site. After hatching, the incubating adult remains with the chick for a number of days, a period known as the guard phase. In the case of most burrow-nesting species, this is only until the chick is able to thermoregulate, usually two or three days. Diving-petrel chicks take longer to thermoregulate and have a longer guard phase than other burrow nesters. However, surface-nesting species, which have to deal with a greater range of weather and to contend with predators like skuas and frigatebirds, consequently have a longer guard phase (as long as two weeks in procellariids and three weeks in albatrosses).
The chick is fed by both parents. Chicks are fed on fish, squid, krill, and stomach oil. Stomach oil is oil composed of neutral dietary lipids that are the residue created by digestion of the prey items. As an energy source for chicks it has several advantages over undigested prey, its calorific value is around 9.6 kcal per gram, which is only slightly lower than the value for diesel oil. This can be a real advantage for species that range over huge distances to provide food for hungry chicks. The oil is also used in defence. All Procellariiformes create stomach oil except the diving-petrels.
The chick fledges between two and nine months after hatching, almost twice as long as a gull of the same body mass. The reasons behind the length of time are associated with the distance from the breeding site to food. First, there are few predators at the nesting colonies, therefore there is no pressure to fledge quickly. Second, the time between feedings is long due to the distance from the nest site that adults forage, thus a chick that had a higher growth rate would stand a better chance of starving to death. The duration between feedings vary among species and during the stages of development. Small feeds are frequent during the guard phase, but afterward become less frequent. However, each feed can deliver a large amount of energy; both sooty shearwater and mottled petrel chicks have been recorded to double their weight in a single night, probably when fed by both parents.