Giant petrel

Lore & Background

Giant petrels, the largest birds in the family Procellariidae, are the only members of this group capable of walking on land. They superficially resemble albatrosses but can be distinguished by their bill structure: the two tube nostrils are joined on top of the bill, whereas in albatrosses they are separated and on the side. Their bills are hooked, composed of seven to nine horny plates, and feature a maxillary unguis to hold slippery prey. The two living species—the southern giant petrel and the northern giant petrel—are difficult to tell apart. Both have long, pale, orange bills and uniform, mottled grey plumage, though about 15% of southern giant petrels are almost completely white. Key differences include bill tip color: reddish-pink in the northern species and pale green in the southern. Adults of the northern species typically have pale eyes, while southern adults usually have dark eyes. The southern giant petrel is slightly larger, with a wingspan of 3,180 millimeters and body length of 86 millimeters, compared to the northern’s 3,150-millimeter wingspan and 80-millimeter body length. An extinct species, *Macronectes tinae*, known from Pliocene New Zealand, had smaller bodies than its living relatives. Giant petrels are extremely aggressive predators and scavengers, inspiring the common name “stinker.” They produce a stomach oil of wax esters and triglycerides, stored in the proventriculus, which can be sprayed as a defense or used as a protein-rich food source for chicks and adults during long flights. A salt gland above the nasal passage excretes a high saline solution to desalinate their bodies.

Reader's Guide

Giant petrels are the largest birds in the family Procellariidae and the only members capable of walking on land, a trait that sets them apart from all other petrels. Their powerful legs allow them to move effectively on solid ground, where they exhibit highly aggressive behavior as both scavengers and predators. They are known to dominate carcasses using a distinctive "sealmaster posture," with head and wings outstretched and tail raised vertically. On land, they feed on carrion from seal and penguin colonies and will kill other seabirds, including albatross-sized birds, by battering or drowning them. At sea, they take krill, squid, and fish, and frequently follow ships to consume offal. This opportunistic feeding strategy, combined with their ability to store protein-rich stomach oil in the proventriculus—which they can spray as a defense or feed to chicks—demonstrates their adaptability. The genus has historically been known by various names from seamen and whalers, including stinker, molly-hawk, gong, glutton bird, and nelly, reflecting their scavenging nature. The extinct species *Macronectes tinae*, known from Pliocene New Zealand, indicates the genus has long been confined to the Southern Hemisphere. Within Procellariidae, giant petrels are grouped with fulmars and other species like the Antarctic and Cape petrels, forming a distinct subgroup. Their size, terrestrial capability, and aggressive dominance of coastal and island food resources make them ecologically significant as top scavengers and predators in Southern Hemisphere ecosystems.

Did You Know?

• Giant petrels are the only procellarids capable of walking on land.
• They can spray a stomach oil of wax esters and triglycerides as a defense.
• A vagrant giant petrel was recorded in the United Kingdom in July 2019, the first in Europe.

A Pliocene Marine Predator

The giant petrel Macronectes tinae is known from the fossil record of New Zealand's southwestern North Island, where its remains were recovered from the mudstones of the Tangahoe Formation. This geologic unit, which stretches across the Taranaki and Manawatū-Whanganui Regions, was laid down beneath sea level during the Waipipian stage of the mid-Pliocene, approximately 3.4 to 3.0 million years ago. The petrel's fossilized remains sit alongside those of other marine organisms, including the megalodon shark, the penguin Eudyptes atatu, the monk seal Eomonachus belegaerensis, and various cetaceans, all drawn from the same muddy sandstone deposits. As a member of the genus Macronectes, Macronectes tinae belongs to the group of birds we recognize today as giant petrels, yet its presence in Pliocene marine sediments tells us it occupied a world very different from the one its modern relatives inhabit. The Tangahoe Formation, named for the nearby Tangahoe River, thus serves as a window into the ancient marine life that once thrived in these waters.

The Tangahoe Formation: A Fossil Treasure Trove

The Tangahoe Formation stands as one of the significant fossil-bearing geologic units in New Zealand. Composed primarily of muddy sandstones, this sedimentary formation was deposited beneath sea level during the Waipipian stage of the mid-Pliocene, spanning a period from roughly 3.4 to 3.0 million years ago. Its geographic footprint is considerable: the formation extends in an east–west band approximately 200 kilometres long, running from the volcanic deposits associated with Mount Taranaki in the west all the way to the Ruahine Ranges in the east, with an average width of about 20 kilometres. At its western terminus, near the Tasman Sea coast close to Hāwera, the formation breaks through the landscape in a series of dramatic cliff faces. It is from these exposed cliffs and the mudstones they contain that paleontologists have extracted a remarkable assemblage of marine fossils, making the Tangahoe a site of enduring scientific importance for understanding Pliocene ocean life.

A Community of Ancient Sea Creatures

The mudstones of the Tangahoe Formation preserve a diverse snapshot of a Pliocene marine community, and the giant petrel Macronectes tinae is just one thread in that tapestry. Alongside the petrel, researchers have identified the remains of the megalodon shark, one of the most formidable predators in the ancient ocean. The penguin Eudyptes atatu and the monk seal Eomonachus belegaerensis further enrich this assemblage, while cetaceans of various kinds add yet another dimension to the ecosystem. Together, these fossils—drawn from the same muddy sandstone deposits laid down 3.4 to 3.0 million years ago beneath the waves—suggest a marine environment teeming with large-bodied animals. The co-occurrence of a giant petrel with such a range of species, from apex sharks to flightless seabirds and large seals, underscores the ecological richness of these Pliocene waters. The Tangahoe Formation thus offers a rare, multi-species portrait of a world that no longer exists, preserved in the sedimentary rocks of New Zealand's North Island.

Reading the Rocks: Geology and Geography

Understanding the giant petrel Macronectes tinae requires understanding the geologic context in which it was preserved. The Tangahoe Formation, from which the petrel's fossils originate, is a sedimentary unit composed largely of muddy sandstones. These sediments accumulated beneath sea level during the Waipipian stage of the mid-Pliocene, a window of time stretching from approximately 3.4 to 3.0 million years ago. The formation occupies a broad east–west corridor across the southwestern North Island of New Zealand, spanning the Taranaki and Manawatū-Whanganui Regions. It stretches roughly 200 kilometres from the volcanic deposits of Mount Taranaki in the west to the Ruahine Ranges in the east, averaging about 20 kilometres in width. At its western edge, near Hāwera and the Tasman Sea coastline, the formation is exposed in a series of cliff faces, and it is in this area that the Tangahoe River flows close by, lending its name to the entire geologic unit.