Diving-petrel

The diving petrels are seabirds in the bird order Procellariiformes. There are four very similar species all in the family Pelecanoididae and genus Pelecanoides (Lacépède, 1799), distinguished only by small differences in the coloration of their plumage and their bill construction.

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. Amongst 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.

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 forward pointing as it is in other tubenoses.

Description and morphology
The diving petrels are small petrels that measure between 19–23 cm (7.5–9 in) and weigh between 120-200 g (4-7 oz). They are highly uniform in appearance, and very difficult to separate when seen at sea. They are best separated 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.

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, whilst the Common Diving Petrel and the South Georgia Diving Petrel range widely across the southern oceans, breeding on islands off New Zealand, sub-Antarctic islands in the Indian Ocean, and islands in the south Atlantic (like Tristan da Cunha).

Diving petrels are among the world's most numerous birds, with Common and South Georgia Diving Petrels numbering several million pairs each. The Peruvian Diving Petrel, on the other hand, is threatened by guano extraction, introduced species and climate change, and is listed as an endangered species.

Systematics and evolution
Some studies published on the phylogeny of the petrels suggests that the diving petrels are actually members of the family Procellariidae, and some taxonomic works treat them as such.

The four species are:
 * Peruvian Diving Petrel Pelecanoides garnotii
 * Magellanic Diving Petrel Pelecanoides magellani
 * South Georgia Diving Petrel Pelecanoides georgicus
 * Common Diving Petrel Pelecanoides urinatrix

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.