Giant petrels are extremely aggressive predators and scavengers, inspiring another common name, the stinker.

[2] Seamen and whalers also referred to the giant petrel as the molly-hawk, gong, glutton bird and nelly.

[4] The genus Macronectes was introduced in 1905 by the American ornithologist Charles Wallace Richmond to accommodate what is now the southern giant petrel.

In July 2019, an individual, either of M. giganteus or M. halli, was found as a vagrant in County Durham and Northumberland in the United Kingdom, marking the first record of the genus in Europe.

This can be sprayed out of their mouths as a defense against predators and as a protein-rich food source for chicks and for the adults during their long flights.

[14] Petrels have a salt gland situated above the nasal passage that helps to desalinate their bodies by excreting a high saline solution from their noses.

[15] The two species are difficult to tell from each other, possessing similar long, pale, orange bills and uniform, mottled grey plumage (except for around 15% of southern petrels, which are almost completely white).

The billtip of M. halli is reddish-pink and that of M. giganteus is pale green, appearing slightly darker and lighter than the rest of the bill, respectively.

[citation needed] The extinct Macronectes tinae is characterized by having smaller bodies than their living relatives.

[16] The southern giant petrel is more likely to form loose colonies than the northern, both species laying a single egg in a rough nest built about 50 cm (20 in) off the ground.