Diving petrel

The diving petrels form a genus, Pelecanoides, of seabirds in the family Procellariidae. There are four very similar species of diving petrels, distinguished only by small differences in the coloration of their plumage, habitat, and bill construction. They are only found in the southern hemisphere. The diving petrels were formerly placed in their own family, the Pelecanoididae.

Diving petrels are auk-like small petrels of the southern oceans. The resemblances with the auks are due to convergent evolution, since both families feed by pursuit diving, although some researchers have in the past suggested that the similarities are due to relatedness. Among the Procellariiformes the diving petrels are the family most adapted to life in the sea rather than flying over it, and are generally found closer inshore than other families in the order.

Taxonomy
The genus Pelecanoides was introduced in 1799 by the French naturalist Bernard Germain de Lacépède for the common diving petrel. The genus name combines the Ancient Greek pelekan  meaning "pelican" and "-oidēs" meaning "resembling".

The diving petrels were formerly placed in their own family, Pelecanoididae. When genetic studies found that they were embedded within the family Procellariidae, the family Pelecanoididae were merged into Procellariidae.

Extant Species
Four species are usually recognised:

A fifth species, the Whenua Hou diving-petrel (Pelecanoides whenuahouensis), is sometimes recognised. It was first described in 2018 and is found around Codfish Island / Whenua Hou, New Zealand. In the bird list maintained by Frank Gill, Pamela Rasmussen and David Donsker on behalf of the International Ornithological Committee the Whenua Hou diving-petrel is treated as a subspecies of the South Georgia diving petrel.

The evolution and systematics of these birds is not well researched. Several populations were described as distinct species and while most of them are only subspecies, some may indeed be distinct. The prehistoric fossil record was long limited to very fragmentary remains described as P. cymatotrypetes found in Early Pliocene deposits of Langebaanweg, South Africa; while this bird apparently was close to the common diving petrel, no members of the genus are known from South African waters today.

In 2007, a humerus piece from New Zealand was described as P. miokuaka. This was found in Early/Middle Miocene deposits and just as may be expected, it far more resembles diving petrels than any other known bird, but presents a less apomorphic condition.

Description
The diving petrels are small petrels that measure between 19 – and weigh 120 –. They are highly uniform in appearance, and very difficult to distinguish when seen at sea. They are best distinguished by the size and shape of their short bills. The plumage is shining black on the top and white on the underside. Their wings are short, particularly with regards to overall body size, and used in a highly characteristic whirring flight. This flight is low over the water and diving petrels will fly through the crests of waves without any interruption of their flight path. In the water these wings are half folded and used as paddles to propel the bird after its prey.

Food and feeding
Diving petrels are plankton feeders, taking mostly crustacean prey such as krill, copepods and the amphipod Themisto gaudichaudii, also taking small fish and squid. They have several adaptations for obtaining their prey including short powerful wings, a gular pouch for storing food, and their nostrils open upwards rather than pointing forward as in other tubenoses.

Breeding
These birds nest in colonies on islands. One white egg is laid in a burrow in turf or soft soil that is usually covered with vegetation, feathers, or small rocks. They are nocturnal at the breeding colonies. It has a long period of parental care (around 45 to 60 days) in the burrow, but once the chick fledges out to sea it is on its own.

Status and conservation
Of the four species, two, the Peruvian diving petrel and the Magellanic diving petrel, have highly restricted ranges around South America's coasts, while the common diving petrel and the South Georgia diving petrel range widely across the southern oceans, breeding on islands off New Zealand, Subantarctic islands in the Indian Ocean, and islands in the south Atlantic (like Tristan da Cunha). The subspecies, the Whenua Hou diving petrel, has an extremely restricted range, breeding only on New Zealand's Codfish Island / Whenua Hou.

Diving petrels are among the world's most numerous birds, with common and South Georgia diving petrels numbering several million pairs each. The Peruvian and Whenua Hou diving petrels, on the other hand, are highly threatened by guano extraction, introduced species and climate change, and are considered endangered species.