Great Shearwater

Birds Name	Great shearwater
Science Name	Ardenna gravis
Domain	Eukaryota
Kingdom	Animalia
Phylum	Chordata
Class	Aves
Order	Procellariiformes
Family	Procellariidae
Genus	Ardenna
Species	A.gravis

The Great Shearwater (Ardenna gravis) represents one of the most compelling biological phenomena in the marine world. A seabird of paradoxical existence, it is at once ubiquitous and elusive; its population numbers in the millions, yet its breeding grounds are restricted to a handful of remote volcanic specks in the South Atlantic Ocean. To the observer on the shores of the Eastern United States, the Great Shearwater is a seasonal visitor, a summer spectacle skimming the troughs of the Atlantic waves. However, to the ornithologist and the marine ecologist, this species is a “globetrotter” of extraordinary endurance, a trans-equatorial migrant that stitches together the hemispheres in a great annual loop of more than 30,000 kilometers.

This report provides a comprehensive, expert-level analysis of the Great Shearwater, designed for a sophisticated audience of birdwatchers, wildlife enthusiasts, and conservationists. It moves beyond basic field guide descriptions to explore the evolutionary history, physiological mastery, complex migration dynamics, and the precarious conservation status of this oceanic wanderer. By synthesizing data from satellite tracking projects, dietary studies, and breeding colony censuses, we illuminate the life history of a bird that spends the vast majority of its life out of sight of land, yet serves as a critical sentinel for the health of our global oceans.

The narrative that follows is structured to guide the reader through every facet of the Great Shearwater’s existence. We begin with its taxonomic standing and evolutionary relationships, proceed through the mechanics of its identification and flight, map its immense distribution, and delve into the intimacies of its breeding biology on the Tristan da Cunha archipelago. Finally, we confront the anthropogenic threats—from industrial fisheries to plastic pollution—that shadow its future. Through this detailed examination, the report underscores the interconnectedness of marine ecosystems, where a bird hatched in a burrow on a sub-Antarctic island becomes a vital component of the food web in the Gulf of Maine.

Taxonomy and Systematics

Evolutionary History and Classification

The taxonomic history of the Great Shearwater reflects the broader evolution of ornithological science, moving from morphological categorization to phylogenetic classification based on genetic divergence. Originally described in 1818 by the Irish naturalist Bernard O’Reilly, the species was given the binomial name Procellaria gravis. The specific epithet gravis, Latin for “heavy” or “weighty,” was an apt descriptor for this robust seabird, distinguishing it from the lighter, more gracile petrels known to mariners of the 19th century.

For much of the 20th century, the Great Shearwater was placed within the genus Puffinus, a “wastebasket” taxon that included a wide variety of shearwater species ranging from the small Manx Shearwater (Puffinus puffinus) to the large Great and Sooty Shearwaters. This classification stood until the advent of advanced molecular phylogenetics in the early 2000s. A pivotal study by Penhallurick and Wink (2004), corroborated by subsequent genomic analyses, revealed that the genus Puffinus was paraphyletic. The genetic distance between the smaller shearwaters and the larger, surface-nesting species was significant enough to warrant a taxonomic split.

Consequently, the Great Shearwater, along with its close relatives the Sooty Shearwater (Ardenna grisea), Short-tailed Shearwater (Ardenna tenuirostris), Pink-footed Shearwater (Ardenna creatopus), Flesh-footed Shearwater (Ardenna carneipes), Buller’s Shearwater (Ardenna bulleri), and Wedge-tailed Shearwater (Ardenna pacifica), was resurrected into the genus Ardenna. This genus name was originally introduced in 1853 by Ludwig Reichenbach, who cited the Italian naturalist Ulisse Aldrovandi’s 1603 use of the term “Artenna” for a seabird. Today, the American Ornithological Society (AOS) and international bodies recognize Ardenna gravis as the valid scientific name, placing it in a clade of large, powerful shearwaters distinct from the smaller, diving-focused Puffinus species.

Relationships within Procellariidae

The Great Shearwater belongs to the family Procellariidae, a diverse group of seabirds known collectively as “tubenoses” due to the external tubular nostrils that sit atop their bills—a critical adaptation for olfaction and salt excretion. Within the genus Ardenna, A. gravis is unique. It is a monotypic species, meaning no subspecies are recognized across its global range. This lack of subspecies differentiation is likely a result of its extreme mobility; with birds from all breeding colonies mixing freely in the North Atlantic during the non-breeding season, genetic flow is maintained, preventing localized isolation that drives speciation.

Phylogenetically, Ardenna gravis is closely related to the Sooty and Short-tailed Shearwaters, forming a group of “sooty-type” shearwaters that are characterized by their large size and long-distance trans-equatorial migrations. However, morphologically, the Great Shearwater’s distinct plumage patterns—specifically its capped appearance and white collar—set it apart from the uniformly dark plumage of its closest kin.

Table 1: Taxonomic Classification of the Great Shearwater

Rank	Name	Notes
Kingdom	Animalia
Phylum	Chordata
Class	Aves
Order	Procellariiformes	The “Tubenoses”
Family	Procellariidae	Petrels, Shearwaters, Prions
Genus	Ardenna	Large shearwaters (formerly Puffinus)
Species	Ardenna gravis	Monotypic (No subspecies)
Synonyms	Puffinus gravis	Used in older literature (pre-2004 split)
Protonym	Procellaria gravis	Described by O’Reilly (1818)

Description and Identification

For the field observer, identifying the Great Shearwater is an exercise in observing structural bulk, flight dynamics, and high-contrast plumage features. It is a large, powerful seabird, significantly heavier than the Manx Shearwater often seen from shore, and structurally distinct from the similarly sized Cory’s Shearwater.

Biometric Profile

The Great Shearwater presents a “bull-necked” and heavy-bellied profile. Biometric data confirms its status as a substantial avian biomass in the pelagic environment. Adults typically measure between 43 and 51 centimeters (17–20 inches) in length, with a wingspan ranging from 100 to 118 centimeters (3.5–4.0 feet). Adult weight varies seasonally, fluctuating between 715 and 950 grams depending on breeding status and fat reserves accumulated during the non-breeding season.

Comparing these metrics to sympatric species is crucial for accurate identification, particularly at sea where size illusions are common.

Table 2: Comparative Biometrics of Atlantic Shearwaters

Species	Length (cm)	Wingspan (cm)	Weight (g)	Key Structural Difference
Great Shearwater (Ardenna gravis)	43–51	100–118	715–950	Heavy body, stiff wings, “bull-necked”
Cory’s Shearwater (Calonectris borealis)	45–56	112–126	800–1,200	Largest, broad wings, heavy yellow bill
Sooty Shearwater (Ardenna grisea)	40–51	94–109	650–950	Streamlined, narrower wings, all-dark
Manx Shearwater (Puffinus puffinus)	30–38	76–89	350–575	Small, rapid wingbeats, cruciform shape

Plumage Characteristics

The plumage of the Great Shearwater is sharply patterned, offering several diagnostic field marks that are visible even at a distance.

• The Cap and Collar: The head pattern is the most striking feature. A distinct blackish-brown cap extends down to the eye, contrasting sharply with white cheeks and a white throat. This cap is separated from the dark brown mantle by a broad, conspicuous white neck collar. This “capped” appearance is unique among the large Atlantic shearwaters and is often visible before other details.

• Upperparts: The mantle, back, and upperwings are a scaly dark grey-brown. The feathers on the back often have pale edgings, creating a scaled effect that is discernible at close range or in photographs. A narrow, U-shaped white band across the uppertail coverts (often called the “horseshoe”) is a critical identification feature when viewing the bird flying away.

• Underparts and the “Belly Patch”: The underparts are predominantly white, but with a characteristic dark smudge or patch on the belly. This patch varies in size and intensity among individuals but is a reliable confirmation mark when the bird banks and reveals its underside. The underwings are white, bordered by dark edges and extensive dark markings at the axillaries (the “armpits”), giving the underwing a “dirty” appearance compared to the pristine white underwing of the Cory’s Shearwater.

Flight Behavior and Molt

The flight of the Great Shearwater is powerful and direct. In calm conditions, it employs a “flap-glide” technique, with stiff, straight wings held perpendicular to the body. This stiffness distinguishes it from the Cory’s Shearwater, which holds its wings in a slightly bowed arch and has a lazier, more fluid flap. In high winds, the Great Shearwater engages in dramatic arcing, wheeling high above the wave crests to catch the wind shear before swooping low into the troughs—a classic dynamic soaring behavior.

Molt patterns also aid in identification. Great Shearwaters undergo a rapid molt of their flight feathers upon arriving in the North Atlantic in early summer (June–July). During this period, they may appear “ragged” or have gaps in their wings. By contrast, Cory’s Shearwaters molt later in the season. Observing a large shearwater with missing primary feathers in June is a strong indicator of Ardenna gravis.

Table 3: Field Identification Matrix (Great vs. Cory’s vs. Sooty)

Feature	Great Shearwater	Cory’s Shearwater	Sooty Shearwater
Overall Impression	Large, sharp contrast, “capped”	Massive, languid, “hooded”	Medium-large, dark, agile
Head Pattern	Black cap, white collar, white cheek	Grey-brown hood merging with throat	Uniform dark chocolate brown
Underparts	White with dark belly smudge	Pure white (no smudge)	Dark (some silvery sheen on wing)
Bill Color	Dark (Black/Grey)	Yellow (with dark tip)	Dark (Black/Grey)
Underwing	White with dark borders & “dirty” pits	White with thin dark borders (clean)	Silvery/white flash on primary coverts
Flight Style	Stiff-winged, rapid, straight wings	Bowed wings, lazy/slow flaps	Stiff, fast, agile, “mini-albatross”
Uppertail	White “horseshoe” band	All dark (no white band)	All dark

Distribution, Range, and Population

The distribution of the Great Shearwater is a tale of two hemispheres. It is a bird of extreme range disparity, possessing a foraging range that encompasses the entire Atlantic Ocean while being tethered to a breeding range that is vanishingly small.

Breeding Range: The South Atlantic Citadels

Breeding is restricted almost entirely to the Tristan da Cunha archipelago and Gough Island, British Overseas Territories located in the central South Atlantic Ocean. These volcanic islands are among the most remote inhabited places on Earth, situated approximately midway between South Africa and South America.

1. Nightingale Island Group: This small group, consisting of Nightingale Island, Middle Island, and Stoltenhoff Island, is the primary global stronghold. Despite Nightingale Island’s diminutive size (approx. 2.6 km²), it supports nearly 2 million pairs of Great Shearwaters. The birds honeycomb the island’s peat soil with burrows, achieving densities that are among the highest for any seabird.

2. Inaccessible Island: Located to the southwest of Tristan, Inaccessible Island supports a similarly massive population, estimated at 2 million pairs. The dense tussock grass here is even thicker than on Nightingale, providing optimal cover for surface nesting where soil depth is insufficient for burrows.

3. Gough Island: Situated 400 km southeast of the Tristan group, Gough Island supports a substantial but varying population, estimated between 600,000 and 3 million pairs. This population is under severe threat from introduced house mice.

4. Falkland Islands (Islas Malvinas): A very small, outlier population breeds on Kidney Island in the Falklands. This represents the only breeding site outside the Tristan/Gough complex.

5. Tristan da Cunha (Main Island): Historically, Great Shearwaters bred on the main island of Tristan. However, human colonization, hunting, and the introduction of rats led to their extirpation as a breeding species there, although a few pairs may persist in inaccessible refuges.

Global Population Estimates

The total global population of the Great Shearwater is estimated at approximately 5 million breeding pairs. When including non-breeding adults (sab-adults) and juveniles, the total number of individuals likely exceeds 15 million. While these numbers suggest a robust species, the concentration of more than 90% of the population on just three small islands (Nightingale, Inaccessible, and Gough) creates a high vulnerability to localized catastrophes such as disease outbreaks, oil spills, or introduced predators.

Table 4: Breeding Population Estimates by Location

Non-Breeding Range and Distribution

Outside the breeding season (breeding occurs during the austral summer, November–May), the entire population conducts a trans-equatorial migration to the North Atlantic.

• Northwest Atlantic: From May through August, Great Shearwaters are most abundant off the eastern coast of North America. They congregate in nutrient-rich shelf waters from Florida north to Newfoundland, Labrador, and Greenland. The Gulf of Maine, Georges Bank, and the Grand Banks are critical staging areas where millions of birds gather to molt and fatten on baitfish.

• Northeast Atlantic: As the boreal summer progresses into autumn (August–October), the population shifts eastward. Large numbers appear off the coasts of the United Kingdom, Ireland, and in the Bay of Biscay. These waters serve as the departure point for their southward migration back to the breeding grounds.

Migration Ecology: The Atlantic Loop

The migration of the Great Shearwater is one of nature’s great odysseys—a “Great Loop” that exploits the prevailing wind systems of the Atlantic basin to facilitate energy-efficient travel over thousands of kilometers.

The Great Circle Route

The migration follows a predictable, clockwise trajectory driven by the Atlantic’s major wind gyres.

1. Northward Exodus (April–May): After the chicks fledge in May, the adults and juveniles leave the South Atlantic colonies. They fly northwest, crossing the equator and moving rapidly along the coast of South America and across the Amazon plume. They ride the southeast trade winds to reach the western North Atlantic.

2. The Boreal Summer Sojourn (June–August): Upon reaching the North Atlantic, the birds settle in the productive waters of the continental shelf. This is the period of peak abundance in US waters. The birds are essentially “summer residents” of the Gulf of Maine, where they undergo their annual molt. The abundance of prey here is critical for regenerating flight feathers and building fat stores.

3. The Eastward Shift (August–October): By late summer, the birds begin to drift eastward, carried by the prevailing westerlies. They become increasingly common in the eastern North Atlantic, off the coasts of Europe and North Africa.

4. Southward Return (October–November): The return journey to the southern hemisphere is typically more easterly than the northward leg. Birds fly down the eastern Atlantic, passing the Canary Islands and Cape Verde, before crossing the equator and arcing back toward the Tristan da Cunha archipelago to begin the breeding cycle anew.

Satellite Tracking Insights

Modern telemetry has revolutionized our understanding of this migration. A landmark tracking project in 2008 attached satellite tags to individual birds (named after Tristan families) to map their journeys. The data revealed staggering distances and individual variability.

• “Swain” covered a total distance of 32,827 km. After leaving the North Atlantic, this bird flew to the coast of Uruguay, then crossed the South Atlantic to the Cape of Good Hope (South Africa), before finally returning to the breeding grounds at Gough Island.

• “Glass” covered 28,125 km, summering off Newfoundland before flying a direct route back to Tristan.

• “Repetto” covered 25,559 km, hugging the South American coast of Argentina before crossing to Tristan.

These tracks underscore the fact that the “Atlantic Loop” is not a rigid highway but a broad corridor. Great Shearwaters are opportunistic nomads, capable of diverting thousands of kilometers to exploit ephemeral food sources off the coasts of Africa or South America before returning to breed.

Table 5: Migration Tracking Case Studies (2008 Project)

Habitat and Oceanography

The Great Shearwater is a pelagic specialist, but its distribution is far from random. It is tethered to specific oceanographic features that drive productivity.

Oceanographic Drivers

• Cold Water Affinity: Despite crossing the tropics, Great Shearwaters are fundamentally birds of cool water. In the South Atlantic, they forage in the Sub-Antarctic Frontal Zone (8–10°C). In the North Atlantic, they associate strongly with the cold, oxygen-rich waters of the Labrador Current and the mixing zones where it meets the warmer Gulf Stream.

• Frontal Systems and Upwelling: The species aggregates at shelf breaks and oceanographic fronts—boundaries between water masses of different temperatures or salinities. These fronts act as biological barriers, concentrating plankton and baitfish. The Great Shearwater’s presence in the Gulf of Maine is directly linked to the bathymetric features (underwater banks and ledges) that force nutrient-rich water to the surface, fueling blooms of phytoplankton and zooplankton.

• Sargassum Association: During their trans-equatorial migration, Great Shearwaters are often observed resting on or foraging near extensive mats of Sargassum seaweed. These floating ecosystems attract small fish and invertebrates, providing critical “refueling stations” in the otherwise oligotrophic (nutrient-poor) waters of the central Atlantic gyres.

The Gulf of Maine: A Critical Nursery

For the US-based observer, the Gulf of Maine and Stellwagen Bank National Marine Sanctuary are the premier habitats for this species. Research indicates that the Gulf of Maine serves as a critical nursery and feeding ground, particularly for sub-adult birds and molting adults. The bathymetry of Stellwagen Bank creates a consistent upwelling system that supports massive schools of Sand Lance (Ammodytes spp.), the primary prey that draws shearwaters to these waters.

Foraging and Feeding Ecology

The Great Shearwater is a versatile predator, employing a range of foraging techniques to exploit the diverse prey available across its vast range.

Diet Composition

The diet of the Great Shearwater is adaptable but focuses on three main categories: fish, cephalopods, and crustaceans.

1. Fish: In the North Atlantic, the Sand Lance (Ammodytes spp.) is the single most important prey item. This small, pencil-like fish forms massive schools and is rich in lipids. Shearwaters also consume Herring, Saury, and Mackerel.

2. Squid: Cephalopods are a primary dietary staple, particularly in the Southern Ocean and during the long migration over deep water. Stomach content analysis of chicks and adults on Tristan da Cunha consistently reveals squid beaks and flesh. The species Gonatus is frequently taken.

3. Crustaceans: Euphausiids (krill) and amphipods are consumed, particularly in areas of high zooplankton productivity like the Bay of Fundy.

4. Offal and Scavenging: Great Shearwaters are notorious scavengers. They aggressively follow fishing vessels (trawlers and longliners) to feed on discarded fish and offal. In these competitive frenzies, they are “fearless,” often diving directly under the stern of moving vessels to secure scraps, competing with gulls and other shearwaters.

Foraging Behaviors

• Plunge Diving: Great Shearwaters are capable of plunging into the water from heights of up to 10 meters (30 feet). Unlike gannets, which rely purely on momentum, shearwaters often use their wings for underwater propulsion (pursuit diving) to chase prey deeper.

• Surface Seizing: They frequently sit on the water surface and dip their heads to grab prey (“surface seizing”) or execute shallow “surface plunges” where they essentially belly-flop onto a school of fish.

• Commensalism with Whales: In the Gulf of Maine, a strong commensal relationship exists between Great Shearwaters and Humpback Whales (Megaptera novaeangliae). The whales use “bubble net” feeding techniques to drive schools of Sand Lance to the surface. The shearwaters anticipate these surfacing events, gathering in anticipation and feeding on the fish concentrated by the whales—an interaction that creates “ephemeral feasts” central to the sanctuary’s food web.

Diving Physiology and Depth

While often viewed as surface feeders, Great Shearwaters are competent divers. Research using capillary depth recorders has revealed their underwater capabilities.

• Maximum Depth: Dives of up to 18.9 meters (62 feet) have been recorded.

• Dive Duration: Most dives are short (average 12 seconds), but they can hold their breath for up to 40 seconds.

• Thermal Adaptation: Interestingly, tracking data shows that birds foraging in cold water (8–10°C) dive deeper and more frequently (mean 74 dives/day) than birds in warmer water (mean 19 dives/day). This suggests that cold-water prey (like krill or deep-water squid) requires active pursuit, whereas warm-water foraging is often opportunistic scavenging.

Table 6: Diving Performance Statistics

Parameter	Cold Water Zone (Sub-Antarctic)	Warm Water Zone (Sub-Tropical)	Maximum Recorded
Mean Dives per Day	74	19	–
Mean Depth (m)	2.1 – 9.2	1.9 – 8.2	18.9
Duration (sec)	Longer (active pursuit)	Shorter (scavenging)	40
Primary Prey	Squid / Krill	Surface Fish / Offal	–

Physiological Adaptations

To survive in the harsh, hyper-saline environment of the open ocean, the Great Shearwater has evolved a suite of remarkable physiological adaptations.

Dynamic Soaring

Like their cousins the albatrosses, Great Shearwaters utilize dynamic soaring to travel vast distances with minimal energy expenditure. This flight technique exploits the wind shear gradient above the ocean surface—the phenomenon where wind speed is lowest at the water’s surface (due to friction) and increases with altitude. The bird climbs into the wind, gaining potential energy from the uplift. It then turns and glides downwind, converting that potential energy into kinetic energy (speed) as it descends into the slower air near the surface. By continuously repeating this cycle—climb, turn, dive, turn—the shearwater extracts energy from the wind itself, allowing it to fly thousands of kilometers without flapping. However, because shearwaters have higher wing loading (weight per wing area) than albatrosses, they must flap more frequently, especially in light winds.

Stomach Oil: High-Octane Aviation Fuel

A unique trait of the order Procellariiformes is the production and storage of stomach oil. This oil is a residue of neutral dietary lipids (wax esters and triglycerides) that accumulates in the proventriculus (foregut).

• Energetics: The oil is essentially “concentrated food.” Digestion separates the water and protein from the prey, leaving the lipids. This reduces the weight the adult must carry while concentrating the caloric value. The energy density of stomach oil (approx. 40 MJ/kg) is nearly that of diesel fuel. This allows adults to forage thousands of kilometers away and return with a high-energy meal for their chick.

• Defense: The oil also serves as a defensive weapon. If threatened on the nest, adults and chicks can forcibly eject the foul-smelling oil at predators. The oil destroys the waterproofing of other birds’ feathers, which can be fatal for avian predators like skuas.

Salt Glands and Osmoregulation

Drinking seawater would be fatal to most terrestrial animals due to the high salt content, which dehydrates cells. Great Shearwaters possess specialized supraorbital salt glands located in grooves in the skull above the eyes. These glands act as “extra kidneys,” actively filtering excess sodium and chloride from the blood. The concentrated saline solution is then excreted through the bird’s tubular nostrils. This adaptation allows the Great Shearwater to remain at sea indefinitely, hydrating solely from the water found in its prey and the seawater it drinks.

Breeding Biology

The breeding cycle of the Great Shearwater is a highly synchronized annual event, dictated by the seasonality of the South Atlantic.

Phenology

• Arrival (September): Birds return to the Tristan da Cunha archipelago in mid-September. This period is marked by intense activity as pairs reunite, reclaim burrows, and engage in courtship.

• Pre-laying Exodus (October): In a phenomenon unique to petrels, the females depart the colony in October for a “pre-laying exodus.” They spend approximately one month at sea, feeding intensively to form the egg. The single egg represents a massive energy investment, weighing about 6-8% of the female’s body mass.

• Laying (November): The females return in early November to lay a single white egg. Laying is remarkably synchronous across the population, peaking around November 13–15.

• Incubation (November–January): Both parents share incubation duties, shifting in long stints of several days to weeks. The incubation period lasts approximately 53–57 days.

• Hatching (January): Chicks hatch in early January. They are initially brooded by the parents but are soon left alone in the burrow while both adults forage at sea.

• Fledging (May): The chick rearing period is long, lasting 105–120 days. Chicks are fed infrequent but massive meals of stomach oil and fish. By May, the chicks are fully grown and often weigh more than their parents. They fledge (leave the nest) and immediately begin their northward migration, independent of the adults.

Table 7: Annual Breeding Cycle (Tristan da Cunha)

Month	Activity Phase	Notes
September	Colony Arrival	Burrow renovation, courtship
October	Pre-laying Exodus	Females at sea (fattening)
November	Egg Laying	Peak laying ~Nov 13-15 (Single egg)
December	Incubation	Shared duties (53-57 days)
January	Hatching	Peak hatching early Jan
Feb – April	Chick Rearing	Adults foraging long-distance
May	Fledging	Migration North begins
June – Aug	Non-Breeding	North Atlantic residence (Molt)

Colony Structure

Great Shearwaters are colonial nesters. On Nightingale Island, they nest in burrows dug into the soft peat soil under Spartina tussock grass. Burrow densities are staggering, reaching up to 10,000 burrows per hectare in prime habitat. The ground is effectively honeycombed, making the island fragile and difficult to traverse without collapsing nests. On Inaccessible Island, where the tussock is even denser, some birds forego digging and incubate their eggs on the surface, protected by the thick canopy of grass—a behavior rare among burrowing petrels.

Threats and Conservation

Despite its abundance, the Great Shearwater faces a “perfect storm” of threats: its breeding is concentrated in a few locations, and its foraging range overlaps with intense human activity.

The Gough Island Catastrophe: Invasive Mice

The most immediate and gruesome threat to the species exists on Gough Island. Here, introduced House Mice (Mus musculus) have evolved into a distinct population of “mega-mice,” growing 50% larger than typical mice. These rodents have learned to attack seabird chicks in their burrows. Video surveillance has captured mice eating Great Shearwater chicks alive. While the impact is catastrophic for the Tristan Albatross, the Great Shearwater population on Gough (estimated at up to 3 million pairs) is also suffering significant chick mortality. Conservation efforts to eradicate mice from Gough are ongoing but face immense logistical challenges.

Plastic Pollution: A Global Sentinel

Great Shearwaters are among the most plastic-contaminated birds in the world. They feed by snatching items from the ocean surface, making them highly prone to mistaking floating plastic fragments for prey.

• Ingestion Rates: Studies in the Northwest Atlantic and South Atlantic have found plastic in 70–95% of necropsied individuals.

• Burden: The average bird contains 9 plastic fragments, but loads can be much higher.

• Toxicology: Research indicates that the ingested plastic leaches toxic chemicals, including polychlorinated biphenyls (PCBs) and plasticizers, into the bird’s tissues.

• Age Vulnerability: Juveniles sampled in the US (Stellwagen Bank) carry higher frequencies of plastic than adults, likely due to inexperience.

• Types: The majority of plastic found is polyethylene (hard fragments), ingested directly from the sea rather than secondarily through fish.

Table 8: Plastic Ingestion Statistics (NW Atlantic vs. South Atlantic)

Metric	NW Atlantic (US/Canada)	South Atlantic (Breeding)
Primary Demographic	Juveniles / Non-breeders	Breeding Adults / Chicks
Frequency of Occurrence	High (>70%)	High (>90%)
Plastic Type	Smaller, hard fragments	Larger, varied plastics
Ingestion Source	Direct Surface Seizing	Direct Surface Seizing
Trend	Mass of plastic increasing over time	High accumulation in chicks

Fisheries Interactions

• Bycatch: Great Shearwaters are frequent victims of fisheries bycatch. They are caught on longline hooks (diving for bait as it is set) and entangled in gillnets. In the US, they are a documented bycatch species in the sink gillnet fishery.

• Competition: Overfishing of key prey species, particularly Sand Lance in the Northwest Atlantic, poses a long-term threat to their food security.

Cultural and Historical Significance

The Great Shearwater has long been interwoven with the human history of the Atlantic.

• “Muttonbirding”: On Tristan da Cunha, the birds (known locally as “Petrels”) were historically a crucial food source. Islanders harvested eggs and chicks for meat and rendered adults for oil (“fatting”). While commercial harvest has ceased, a regulated traditional harvest continues on Nightingale Island, maintaining a cultural link between the islanders and the birds.

• Maritime Folklore: To the fishermen of the Grand Banks, Great Shearwaters were known as “Hagdowns” or “Hagdens,” likely a reference to “Hag” (witch) due to their dark caps and the eerie, wailing calls they make at night on the breeding grounds—sounds that early mariners found superstitious and unsettling.

Conclusion

The Great Shearwater is a master of the Atlantic, a bird whose life history maps the contours of the ocean basin. From the peat burrows of Nightingale Island to the rich upwellings of the Gulf of Maine, it connects hemispheres and ecosystems. Its ability to navigate 30,000 kilometers a year, dive to depths of 19 meters, and thrive in the fiercest winds on the planet is a testament to the evolutionary perfection of the Procellariiformes.

However, the Great Shearwater is also a warning. The plastic found in the stomach of a chick on Gough Island, the mouse attacking it in its burrow, and the gillnet waiting in the Gulf of Maine are reminders that even the most remote ocean wanderers are not beyond our reach. Ensuring that the “Hagdown” continues to wheel over the Atlantic waves requires not just local protection of its breeding islands, but a global commitment to a cleaner, safer ocean.