Great Shearwater

Characterized by a robust and streamlined silhouette, this member of the Procellariidae family, scientifically known as Ardenna gravis, measures 43 to 51 centimeters (17 to 20 inches) in length and boasts a wingspan of 100 to 118 centimeters (39 to 46 inches). It features a highly contrasting plumage, defined by a stark dark brown cap that extends to the level of the eyes, sharply demarcated by a prominent white collar across the nape of the neck. The upperparts are predominantly grayish-brown with pale feather fringes that create a scaly effect, while the underparts are mostly clean white, except for a distinct smudge of brown on the central belly and dark markings on the underwing linings. In flight, a diagnostic crescent-shaped white horseshoe patch on the upper tail-coverts is highly visible against the dark tail. The bill is long, slender, hooked, and blackish-gray in coloration, perfectly adapted for seizing slippery marine prey.

Geographical distribution: Exhibiting a massive transequatorial migratory loop, its breeding grounds are restricted to the southern hemisphere, specifically concentrated in the Tristan da Cunha archipelago, Gough Island, and minimally on the Falkland Islands. Following the breeding season, it embarks on a spectacular clockwise migration across the Atlantic Ocean, traveling north along the western corridor to reach the highly productive sub-Arctic waters of Newfoundland, Greenland, and Iceland during the boreal summer. The return journey to the South Atlantic is undertaken primarily via the eastern Atlantic route, completing an extraordinary annual circuit of tens of thousands of kilometers over the open ocean.

Habitat: This is a strictly pelagic species that spends the vast majority of its life cycle far out at sea, rarely approaching land except during severe oceanic gales or when nesting. It shows a pronounced affinity for upwelling zones, thermal fronts, and cold, nutrient-rich oceanic currents where marine productivity is exceptionally high. It is commonly found over the continental shelf edge and deep offshore waters. The species only transitions to a terrestrial environment on remote oceanic islands characterized by steep slopes, volcanic soils, and dense cover of tussock grasses or low subantarctic vegetation.

Feeding: Primarily a carnivorous and opportunistic feeder, its diet consists of small schooling fish such as capelin and sand lance, as well as various species of squid and pelagic crustaceans like krill. It employs versatile foraging techniques, including pursuit-diving from the air or the surface, flapping its wings underwater to propel itself to depths exceeding 10 meters (33 feet). Additionally, it is a highly gregarious scavenger that frequently follows commercial fishing vessels to feed on discarded offal and fish scraps.

Behavior: Highly gregarious at sea, it frequently gathers in massive rafts of thousands of individuals over rich feeding grounds, often associating with marine mammals and other seabird species. Its flight is exceptionally graceful and efficient, characterized by a series of rapid wingbeats followed by long, sweeping dynamic soaring glides mere inches above the wave crests, exploiting wind shear. On land, its locomotion is clumsy and awkward due to the far-back placement of its legs, prompting it to adopt a strictly nocturnal habit at breeding colonies to evade predation by predatory skuas and gulls.

Nesting: The breeding season commences between October and November when adults return to their subantarctic nesting colonies. It nests in burrows excavated in soft soil, turf, or beneath dense tussock grass clumps, which are dug by both partners using their feet and bills. A single white egg is laid, and both parents share incubation duties for approximately 55 days in alternating shifts. Once hatched, the chick is fed a rich mixture of regurgitated fish, squid, and highly concentrated stomach oil, fledging after 105 to 120 days before venturing into the open ocean as fully independent juveniles.

Conservation status: It is currently classified as Least Concern (LC) on the IUCN Red List due to its immense global population, estimated at several million individuals. However, its status requires vigilant monitoring because almost the entire global population is concentrated on a few highly restricted island groups, making them vulnerable to localized environmental catastrophes. Principal threats include bycatch in longline and driftnet fisheries, the ingestion of marine microplastics, and habitat degradation caused by introduced predators, particularly invasive mice on Gough Island which prey heavily on helpless chicks.