Great Shearwater

Ardenna gravis

Order: Procellariiformes / Family: Procellariidae

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brown white large seabird coastal migratory common widespread colonial

About the Great Shearwater

The Great Shearwater, known scientifically as Ardenna gravis, is a remarkable seabird that captivates birders and ocean enthusiasts alike with its striking appearance and remarkable adaptability to marine environments. This bird, belonging to the family Procellariidae, is a medium to large-sized seabird characterized by its long wings and sleek body, which enable it to excel in dynamic oceanic conditions. The Great Shearwater is an important member of pelagic ecosystems, often seen gliding effortlessly over the open seas of the North and South Atlantic. This bird plays a crucial role in the marine food web, acting as both predator and prey in its ecological niche.

The Great Shearwater primarily inhabits the vast expanses of the open ocean, where it is seldom seen by land-dwellers. However, during certain times of the year, this seabird can be observed closer to shorelines, particularly during its breeding season when it nests on remote islands in the Southern Hemisphere. The Great Shearwater's breeding grounds include islands in the South Atlantic such as Gough Island and the Tristan da Cunha archipelago, where it nests in large colonies. These islands provide a safe haven away from mainland predators, allowing the birds to raise their young in relative security.

With its wide-ranging migratory patterns, the Great Shearwater is a symbol of the interconnectedness of marine ecosystems across continents. After the breeding season, these birds embark on a remarkable journey to the North Atlantic, traveling thousands of miles to feed in nutrient-rich waters. This migration not only demonstrates the bird's adaptability but also underscores the importance of international cooperation in marine conservation efforts. The Great Shearwater's presence in both hemispheres highlights its significance as an indicator species for ocean health, making it a focal point for studies on climate change and marine biodiversity.

The Great Shearwater belongs to the family Procellariidae, within the order Procellariiformes. These birds typically live average lifespan is around 20 years, with a maximum recorded of over 30 years..

How to Identify a Great Shearwater

The Great Shearwater is easily recognized by its distinctive plumage and flight pattern, making it a favorite among birdwatchers in pelagic trips. Adult Great Shearwaters display a striking contrast between their dark upperparts and white underparts. The wings are dark above with a characteristic white band on the upper wing coverts, which is visible in flight. The bird's tail is dark with a prominent white rump, a key feature that aids in identification. In terms of size, the Great Shearwater is among the larger shearwaters, with a body length ranging from 43 to 51 centimeters and a wingspan between 105 and 122 centimeters.

Male and female Great Shearwaters exhibit similar plumage, making it difficult to differentiate between the sexes in the field. Juveniles resemble adults but may appear slightly duller in color. One of the challenges in identifying the Great Shearwater lies in distinguishing it from similar species such as the Sooty Shearwater or the Cory's Shearwater. However, the Great Shearwater's distinctive white underparts and unique flight pattern, which includes long glides interspersed with rapid flapping, set it apart.

When observed at sea, the Great Shearwater's flight silhouette is unmistakable, with long narrow wings held stiffly as it skims the water's surface. Its perching posture, often seen when resting on the water, is upright and alert, allowing for a clear view of its coloration. This bird's presence is often announced by its graceful gliding flight, a skill honed for life on the open ocean.

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Great Shearwater Habitat and Range

Preferred Habitat

The Great Shearwater thrives in the vast and challenging environment of the open ocean, a testament to its adaptability and resilience. It primarily inhabits pelagic zones, ranging from temperate to sub-polar waters. During the breeding season, it is found on remote islands in the South Atlantic such as Gough Island and Tristan da Cunha, where it nests in burrows on grassy slopes and cliff edges. These islands provide the isolation necessary for successful breeding, far from the reach of terrestrial predators.

Outside the breeding season, the Great Shearwater is a highly migratory species, traversing the Atlantic Ocean to exploit different marine habitats. It can be found in cooler waters of the North Atlantic during the austral winter, particularly off the coasts of Newfoundland, Iceland, and the British Isles. These regions provide rich feeding grounds due to the convergence of ocean currents and upwelling zones that bring nutrients to the surface.

While primarily a pelagic species, the Great Shearwater occasionally ventures closer to shorelines, especially during migration. However, it generally avoids urban areas and is rarely seen inland. Its preference for expansive oceanic habitats underscores its role as a true seabird, adapted to the dynamic and often harsh conditions of the open sea.

Geographic Range

The Great Shearwater boasts an impressive range that spans both hemispheres, illustrating its adaptability and endurance. During the breeding season, it is found on remote islands in the Southern Hemisphere, particularly in the South Atlantic. Key breeding sites include Gough Island and the Tristan da Cunha archipelago, where thousands of pairs congregate to nest in colonies.

After breeding, the Great Shearwater undertakes a remarkable migration to the North Atlantic, traveling thousands of kilometers. This journey takes them along the eastern seaboard of North America, across the Atlantic to the waters off the British Isles and Iceland. These areas serve as crucial feeding grounds where the birds exploit abundant fish stocks and marine resources.

The Great Shearwater's migration routes are dictated by ocean currents and prey availability, often leading them across international waters. Their presence in both the North and South Atlantic makes them a key species for monitoring ocean health and understanding the impact of climate change on marine migratory patterns. Population density hotspots include the waters off Newfoundland, where large flocks can be observed feeding during the austral winter.

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What Does the Great Shearwater Eat?

The Great Shearwater is an opportunistic feeder, adept at exploiting a variety of marine food sources. Its diet primarily consists of small fish and squid, which it catches near the ocean surface. During the breeding season, when the birds are anchored to their nesting sites, they rely heavily on locally abundant prey species such as lanternfish and krill.

As a highly migratory species, the Great Shearwater's diet varies with location and season. In the North Atlantic, they take advantage of the rich feeding grounds provided by the convergence of ocean currents. Here, herring and capelin are common prey items. The shearwaters employ various foraging techniques, including plunge-diving, surface-seizing, and pursuit-diving, exhibiting remarkable flexibility depending on prey availability.

Prey size for the Great Shearwater typically ranges from small fish under 15 centimeters to larger squid. Feeding often occurs in large flocks, where the birds capitalize on the presence of predatory fish that drive prey to the surface. This communal feeding behavior not only increases their foraging success but also highlights their role in the marine food web as both predator and competitor.

Great Shearwater Behavior and Social Structure

The Great Shearwater exhibits fascinating social and behavioral traits that reflect its adaptation to life on the open ocean. These birds are highly social, often seen in large flocks, especially during feeding. Their social structure is cooperative, as flocking not only aids in locating food sources but also provides protection against predators such as skuas and gulls.

Territorial behavior in Great Shearwaters is most evident during the breeding season, when pairs defend their nesting burrows from intruders. Courtship displays are modest, with pairs engaging in mutual preening and synchronized flight. These displays reinforce pair bonds, which are typically monogamous for the breeding season.

The Great Shearwater's daily activity patterns revolve around feeding and resting. They are primarily diurnal, with peak activity occurring at dawn and dusk when fish and squid are most active near the ocean surface. In response to predators, Great Shearwaters rely on their agility in flight and the safety of numbers in flocks. They are known to exhibit evasive maneuvers and can dive below the surface to escape threats. Their behavior illustrates a highly adapted lifestyle for survival in the challenging marine environment.

Great Shearwater Nesting and Reproduction

The nesting habits of the Great Shearwater reveal much about its life cycle and survival strategies. Nesting occurs in large, dense colonies on remote islands in the South Atlantic, where the isolation helps protect against many terrestrial predators. The birds prefer burrows dug into the soil on grassy slopes, although some may nest in rock crevices or shallow depressions. These burrows provide shelter from the elements and a secure environment for rearing chicks.

A typical clutch consists of a single egg, which underscores the species' strategy of investing significant parental care in each offspring. Both male and female Great Shearwaters participate in incubation, which lasts approximately 53 to 55 days. This shared responsibility ensures that one parent is always available to guard the nest and maintain egg temperature.

Once hatched, the chick remains in the nest for about 100 to 110 days before fledging. During this period, parents continue to provide food, mainly regurgitated fish and squid. After fledging, young shearwaters embark on their first migration to the North Atlantic, a journey that marks their transition to independence. The Great Shearwater's nesting strategy, characterized by long-term parental care and protection, highlights its adaptation to the challenges of seabird life.

Great Shearwater Sounds and Vocalizations

The vocalizations of the Great Shearwater contribute to its enigmatic presence in the pelagic zone. While generally more silent compared to land birds, Great Shearwaters produce a variety of calls, especially on their breeding grounds. Their primary vocalization is a series of low, rhythmic croaks and wails, often heard during night-time when birds are most active around their nesting colonies. These sounds serve as contact calls between mates and are crucial for maintaining pair bonds and coordinating parental duties.

In flight or at sea, the Great Shearwater is relatively quiet, though it may emit harsh, guttural calls during feeding frenzies or when startled by predators. These calls serve to maintain group cohesion and alert other flock members to potential threats.

Their vocal repertoire lacks the complexity of many land birds, reflecting their need for simple, effective communication across the vast expanses of the ocean. The Great Shearwater's calls are best compared to the low, haunting sounds of distant foghorns, adding to the bird's mystique as a creature of the sea.

Interesting Facts About the Great Shearwater

1The Great Shearwater migrates over 10,000 kilometers annually, showcasing remarkable endurance and navigational skills.
2Great Shearwaters can dive up to 10 meters below the ocean surface to catch prey, demonstrating their adept foraging abilities.
3These birds are one of the few seabird species that inhabit both the Northern and Southern Hemispheres during different times of the year.
4A Great Shearwater colony on Gough Island can number in the tens of thousands, highlighting their social and colonial nature.
5During migration, Great Shearwaters can cover more than 900 kilometers in a single day, aided by strong ocean winds.
6Despite their large size, Great Shearwaters are incredibly agile in the air, capable of rapid changes in direction to evade predators.
7The white rump of the Great Shearwater serves as a key identification feature, even when seen at great distances.

Great Shearwater Conservation Status

The Great Shearwater is currently classified as Least Concern by the IUCN, thanks to its large global population and extensive range. However, like many seabirds, it faces several threats that could impact its long-term survival. These threats include climate change, which affects ocean currents and prey availability, and bycatch in fisheries, where shearwaters can become entangled in fishing gear. Conservation efforts focus on protecting breeding colonies from introduced predators and human disturbance. On islands like Tristan da Cunha, measures include habitat restoration and predator control to safeguard nesting sites. Monitoring programs track population trends and migration patterns to assess the impacts of environmental changes. Historically, Great Shearwater populations have experienced fluctuations due to hunting and egg collection. Current conservation strategies aim to maintain stable populations by addressing both direct and indirect threats, while international cooperation ensures the protection of this species across its migratory range.