Southern storm-petrel

Austral storm petrels, or southern storm petrels, are seabirds in the family Oceanitidae, part of the order Procellariiformes. These smallest of seabirds feed on planktonic crustaceans and small fish picked from the surface, typically while hovering. Their flight is fluttering and sometimes bat-like.

Austral storm petrels have a cosmopolitan distribution, being found in all oceans, although only the Wilson's storm-petrels are found in the northern hemisphere. They are almost all strictly pelagic, coming to land only when breeding. In the case of most species, little is known of their behaviour and distribution at sea, where they can be hard to find and harder to identify. They are colonial nesters, displaying strong philopatry to their natal colonies and nesting sites. Most species nest in crevices or burrows and all but one species attend the breeding colonies nocturnally. Pairs form long-term monogamous bonds and share incubation and chick-feeding duties. Like many species of seabird, nesting is highly protracted with incubation taking up to 50 days and fledging another 70 days after that.

Several species of austral storm petrel are threatened by human activities. One species, the New Zealand storm petrel was presumed extinct until rediscovered in 2003. The principal threats to storm petrels are introduced species, particularly mammals, in their breeding colonies; many storm petrels habitually nest on isolated mammal-free islands and are unable to cope with predators like rats and feral cats.

Taxonomy
Two subfamilies of storm petrel were traditionally recognized. The Oceanitinae, or austral storm-petrels, were mostly found in southern waters (though the Wilson's storm petrel regularly migrates into the northern hemisphere); there were seven species in five genera. The Hydrobatinae, or northern storm petrels, were the two genera Hydrobates and Oceanodroma. They were largely restricted to the northern hemisphere, although a few could visit or breed a short distance beyond the equator.

Cytochrome b DNA sequence analysis suggests that the family was paraphyletic and more accurately treated as distinct families. The same study found that the austral storm petrels are basal within Procellariiformes. The first split was the subfamily Oceanitidae, with the Hydrobatidae splitting from the rest of the order at a later date. Few fossil species have been found, with the earliest being from the Upper Miocene.

Morphology and flight
Austral storm petrels are the smallest of all the seabirds, ranging in size from 15–26 cm in length. There are two body shapes in the family; the austral storm petrels have short wings, square tails, elongated skulls, and long legs. The legs of all storm petrels are proportionally longer than those of other Procellariiformes, but they are very weak and unable to support the bird's weight for more than a few steps.

The plumage of the Oceanitidae is dark with white underparts (with the exception of the Wilson's storm petrel)   Onley and Scofield (2007) state that much published information is incorrect, and that photographs in the major seabird books and websites are frequently incorrectly ascribed as to species. They also consider that several national bird lists include species which have been incorrectly identified or have been accepted on inadequate evidence.

Storm petrels use a variety of techniques to aid flight. Most species will occasionally feed by surface pattering, holding and moving their feet on the water's surface while holding steady above the water. They remain stationary by hovering with rapid fluttering or by using the wind to anchor themselves in place. This method of feeding flight is most commonly used by austral storm petrels. 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. Storm petrels also use dynamic soaring and slope soaring to travel over the ocean surface, although this method is used less by this family compared to the northern storm petrels. Slope soaring is more straightforward and favoured by the Oceanitidae, the storm petrel turns to the wind, gaining height, from where it can then glide back down to the sea.

Diet
The diet of many storm petrels species is poorly known owing to difficulties in researching; overall the family is thought to concentrate on crustaceans. Small fish, oil droplets and molluscs are also taken by many species. Some species are known to be rather more specialised; the grey-backed storm petrel is known to concentrate on the larvae of goose barnacles.

Almost all species forage in the pelagic zone, except for the Elliot's storm petrels which are coastal feeders in the Galapagos Islands. Although storm petrels are capable of swimming well and often form rafts on the water's surface they do not feed on the water. Instead feeding usually takes place on the wing, with birds hovering above or "walking" on the surface (see morphology) and snatching small morsels. Rarely prey is obtained by making shallow dives under the surface.

Like many seabirds storm petrels will associate with other species of seabird and marine mammal species in order to help obtain food. It is theorised that they benefit from the actions of diving predators such as seals and penguins which push prey up towards the surface while hunting, allowing the surface feeding storm petrels to reach them.

Distribution and movements
The austral storm-petrels typically breed found in the southern hemisphere, in contrast to the Northern storm-petrel in the northern hemisphere.

Several species of storm petrel undertake migrations after the breeding season. The most widely travelled migrant is the Wilson's storm petrel which after breeding in Antarctica and the subantarctic islands regularly crosses the equator to the waters of the north Pacific and Atlantic. Some species, like the grey-backed storm petrel are thought to be essentially sedentary and do not undertake any migrations away from their breeding islands.

Breeding
Storm petrels nest colonially, for the most part on islands, although a few species breed on the mainland, particularly Antarctica. Nesting sites are attended nocturnally in order to avoid predators. Storm petrels display high levels of philopatry, returning to their natal colonies to breed. In one instance a band-rumped storm petrel was caught as an adult 2 m from its natal burrow. Storm petrels nest either in burrows dug into soil or sand, or in small crevices in rocks and scree. Competition for nesting sites is intense in colonies where storm petrels compete with other burrowing petrels, with shearwaters having been recorded killing storm petrels in order to occupy their burrows. Colonies can be extremely large and dense; 840,000 pairs of white-faced storm petrel nest on South East Island in the Chatham Islands in burrow densities of between 1.18 – 0.47 burrows/m2.

Storm petrels are monogamous and form long-term pair bonds that last a number of years. As with the other Procellariiformes, a single egg is laid by a pair in a breeding season; if the egg fails, then usually no attempt is made to re-lay (although it happens rarely). Both sexes incubate in shifts of up to six days. The egg hatches after 40 or 50 days; the young is brooded continuously for another 7 days or so before being left alone in the nest during the day and fed by regurgitation at night. Meals fed to the chick weigh around 10–20% of the parent's body weight, and consist of both prey items and stomach oil. Stomach oil is an energy rich (its calorific value is around 9600 calories per gram) oil created by partly digested prey in a part of the foregut known as the proventriculus. By partly converting prey items into stomach oil storm petrels can maximise the amount of energy chicks receive during feed, an advantage for small seabirds that can only make a single visit to the chick during a 24-hour period (at night). The average age at which chicks fledge depends on the species, taking between 50 or 70 days. The time taken to hatch and raise the young is long for the bird's size but is typical of seabirds, which in general are K-selected, living much longer, delay breeding for longer, and invest more effort into fewer young. The young leave there burrow at about 62 days. They are independent almost at once and quickly disperse into the ocean. They return to their original colony after 2 or 3 years, but will not breed until at least 4 years old. Storm petrels have been recorded living as long as 30 years.

Threats and conservation
Several species of austral storm petrel are threatened by human activities. The New Zealand storm petrel, are listed as critically endangered, and was also considered extinct for many years but was sighted again in 2003, though the population is likely to be very small. Storm petrels face the same threats as other seabirds; in particular they are threatened by introduced species.