Cory’s Shearwater

To the uninitiated observer scanning the Atlantic horizon, the shearwater is a ghost, a fleeting shape vanishing between the swells. But to the seasoned birder standing on the deck of a pelagic vessel off Cape Hatteras or the blustery cliffs of the Azores, the Cory’s Shearwater (Calonectris borealis) is a creature of commanding presence. It is the giant among the North Atlantic’s common tubenoses, a bird that combines the raw power of an albatross with the agile, searching flight of a predator. For the wildlife enthusiast in the United States, encountering a Cory’s Shearwater is often the highlight of a pelagic trip, offering a glimpse into a life lived almost entirely on the wing in the vast, blue wilderness of the open ocean.   

The physical architecture of Calonectris borealis is a masterclass in marine adaptation. It is a large, heavy-bodied bird, significantly more robust than the frenetic Manx Shearwaters or the delicate storm-petrels that often share its waters. Individuals typically measure between 48 and 56 centimeters in length, but it is their wingspan—stretching from 113 to 124 centimeters—that truly defines their silhouette. This impressive span supports a flight style characterized by long, languid glides interspersed with slow, deep, and deliberate wingbeats, a stark contrast to the rapid, flickering flight of smaller shearwater species.   

The plumage of the Cory’s Shearwater is a study in counter-shading, a ubiquitous camouflage strategy among open-ocean predators that helps them blend into the dark sea when viewed from above and the bright sky when viewed from below. The upperparts are a warm, uniform grayish-brown, which can appear to shift in tone depending on the ambient light and the wear of the feathers. In fresh plumage, the feathers may show subtle pale edgings, giving the back a slightly scaled appearance. The underparts are pristine white, extending from the chin and throat down to the undertail coverts. A critical field mark for identification is the underwing pattern: it is predominantly white but bordered by thick, dark margins on the leading and trailing edges. Unlike the closely related Scopoli’s Shearwater, which often shows white “fingers” extending into the dark primary tips, the Cory’s Shearwater typically presents a more solid, dark wingtip, though this feature can be variable and challenging to discern in the field.   

Perhaps the most charismatic feature of the bird is its bill. It possesses a robust, thick, pale yellow or horn-colored bill that stands out beacon-like against the darker head. The bill is tipped with a dark, often blackish, unguis (hook), designed for seizing slippery prey. Surmounting the bill are the tubular nostrils, or naricorns, which give the order Procellariiformes its common name: tubenoses. These tubes are not mere decorations; they are sophisticated biological instruments housing highly developed olfactory organs that allow the bird to smell prey and navigate across featureless oceans. They also function as the exit nozzle for the bird’s internal desalination system. The supraorbital salt glands, located above the eyes, filter excess salt from the bloodstream—a necessity for a creature that drinks seawater—and excrete it as a concentrated brine solution that drips or is sneezed out through the tubes.   

Table 1: Biometric Comparison of Calonectris Shearwaters

The following table contrasts the physical measurements of Calonectris borealis with its closest relatives, highlighting the size dominance of the Atlantic Cory’s Shearwater compared to the Mediterranean and Cape Verde species.

   

Taxonomy

The taxonomic history of the Cory’s Shearwater is a fascinating narrative of scientific refinement, illustrating how modern tools have allowed us to see biodiversity that was previously hidden in plain sight. For decades, the large shearwaters of the eastern Atlantic and Mediterranean were lumped into a single polytypic species, Calonectris diomedea. Under this older classification, the bird we now know as Cory’s Shearwater was merely a subspecies (C. d. borealis).   

However, the application of molecular genetics and bioacoustics has rewritten this lineage. Extensive research analyzing mitochondrial DNA (specifically the cytochrome b gene) and allozymes revealed deep genetic rifts between the populations. The Atlantic borealis and the Mediterranean diomedea are now understood to be distinct sister taxa that diverged approximately one million years ago, likely separated by the fluctuating sea levels and environmental barriers of the Pleistocene glacial cycles. Consequently, the Cory’s Shearwater (Calonectris borealis) was elevated to full species status, separate from Scopoli’s Shearwater (Calonectris diomedea) and the much smaller, darker Cape Verde Shearwater (Calonectris edwardsii).   

This split is not merely academic; it is supported by distinct behavioral and morphological differences. While borealis and diomedea look superficially similar, their vocalizations act as a powerful reproductive barrier. The calls of C. borealis are significantly lower-pitched and follow a different rhythmic structure than the higher, faster calls of C. diomedea. These vocal differences are crucial in preventing hybridization, especially in the few areas where their ranges might abut, such as near the Strait of Gibraltar or the Chafarinas Islands. The Cape Verde Shearwater is the most divergent of the trio, having likely separated even earlier, and is easily distinguished by its black bill and significantly smaller size.   

Distribution, Range and Population

The Cory’s Shearwater is an Atlantic endemic during its breeding season, with a distribution that is tightly centered on the Macaronesian archipelagos. While they roam the entire ocean, their reproductive life is tethered to a few specific island groups in the Northeast Atlantic.

Breeding Range

The global stronghold for the species is the Azores archipelago. This Portuguese territory is home to approximately 60% to 75% of the world’s population, with colonies established on every island and numerous offshore islets. Moving south, significant populations breed in the Madeira archipelago (including Porto Santo and the Desertas Islands) and the Canary Islands. The Savage Islands (Selvagens), a tiny, uninhabited Portuguese nature reserve located between Madeira and the Canaries, hosts the single largest colony in the world on Selvagem Grande. A smaller, peripheral population exists on the Berlengas islands off the coast of mainland Portugal.   

Non-Breeding Range and US Occurrence

When the breeding season concludes in late autumn, Calonectris borealis undertakes a massive trans-equatorial migration. They disperse widely across the Atlantic Ocean. Their wintering grounds are concentrated in nutrient-rich upwelling zones:

• The Benguela Current: Off the coasts of Namibia and South Africa.   

• The Agulhas Current: Extending into the Indian Ocean.   

• The Brazil Current: Off the coast of Brazil and Uruguay.   

For birdwatchers in the United States, the Cory’s Shearwater is a regular and expected visitor to the pelagic waters of the East Coast. From May through October, they forage extensively off the Atlantic Seaboard, from Florida north to Massachusetts. They are particularly drawn to the warm waters of the Gulf Stream and the productive shelf-break fronts. Research has shown that Cory’s Shearwaters are the only seabird species exhibiting a statistically significant relationship to the Gulf Stream surface thermal front, where temperature and salinity gradients aggregate prey.   

Population Statistics

The global population is robust, estimated at over 500,000 mature individuals. While the species faced historical declines due to human exploitation for food and feathers, strict protection measures in the last few decades have allowed for stabilization and even recovery in key areas like the Selvagens.   

Table 2: Estimated Breeding Populations by Archipelago

The following data aggregates colony census counts to illustrate the distribution of the species.

  

Note: Population numbers are estimates based on available census data from varying years (2005-2015) and represent the scale of colonies rather than real-time precise counts.

Habitat

The life of a Cory’s Shearwater is split between two distinct realms: the terrestrial fortress where they breed and the oceanic wilderness where they feed.

Terrestrial Breeding Habitat

As obligate cavity nesters, Cory’s Shearwaters require shelter to protect their single egg and vulnerable chick from the elements and aerial predators like gulls. They favor barren, offshore islands and inaccessible cliffs where terrestrial predators are naturally absent.   

• Geology: They are highly adaptable in their choice of microsites, utilizing natural rock crevices, deep volcanic tubes (lava tunnels), and scree slopes. In the Azores, nesting density is positively correlated with the surface area of available cliffs.   

• Burrowing: Where soil depth permits, they will excavate their own earthen burrows. These burrows are often reused for many years, a testament to the high site fidelity of the species.

• Artificial Adaptation: Interestingly, they have shown a remarkable ability to adapt to human-altered landscapes. In areas where natural cavities are limited, they readily occupy artificial nest boxes, stone walls, and even abandoned buildings or ruins.   

Marine Foraging Habitat

At sea, the Cory’s Shearwater is not randomly distributed; it is an oceanographic specialist.

• Upwelling & Fronts: They are heavily associated with boundary currents and upwelling systems. During the breeding season, they exploit the Canary Current. During winter, they target the Benguela Current. These systems bring cold, nutrient-rich water to the surface, fueling the phytoplankton blooms that support their prey.   

• The Gulf Stream Connection: In the Northwest Atlantic, their abundance peaks at the Gulf Stream front. This specific thermal and salinity boundary concentrates baitfish and squid, creating a predictable foraging ground that attracts birds from thousands of miles away.   

• Bathymetry: While pelagic, they often forage over the continental shelf break and seamounts, where subsurface topography forces currents upward, aggregating prey near the surface.   

Behavior

The behavior of Calonectris borealis is defined by its mastery of the wind and its distinct, often raucous, social rhythms.

Dynamic Soaring and Flight Physics

The flight of the Cory’s Shearwater is a marvel of bio-engineering. Like albatrosses, they utilize dynamic soaring, a technique that allows them to extract energy from the wind shear—the gradient in wind speed between the ocean surface and the air above. By climbing into the wind (gaining height) and turning downwind (gaining ground speed), the bird can cover vast distances with minimal metabolic cost.   

However, unlike the wandering albatross, the Cory’s Shearwater faces a trade-off. Research has shown that while prioritizing energy harvest (pure soaring) is efficient, it can slow down directional progress. To compensate, Cory’s Shearwaters employ a “flap-gliding” strategy. They intersperse long glides with slow, deep wingbeats, allowing them to maintain a direct course even in lighter winds where a pure soarer might be stalled. This “step-selection” strategy allows them to balance energy efficiency with the need to reach specific foraging grounds quickly.   

Rafting and Nocturnal Activity

During the breeding season, the colonies operate on a vampire-like schedule. To avoid predation by Yellow-legged Gulls and diurnal raptors, Cory’s Shearwaters are strictly nocturnal on land. In the late afternoon, thousands of birds gather on the sea surface near the breeding islands, forming vast “rafts.” They wait for civil twilight to fade before making landfall. This rafting behavior is intensely social; the air fills with their calls as they prepare to ascend the cliffs.   

Vocalization and the “Cagarro”

The auditory landscape of a Cory’s Shearwater colony at night is legendary in the Azores. Their call is a guttural, wailing series of notes, often phonetically described as “ia-gowa” or “ca-garro.” This sound is so ubiquitous that “Cagarro” has become the Portuguese name for the bird.

• Sexual Dimorphism in Sound: The calls are sexually dimorphic, allowing researchers to sex birds without handling them. Males produce a lower-pitched, clearer whistling quality (often described as the “breath-in” sound), while females produce a harsher, grating, lower-frequency call (the “breath-out” sound).   

Feeding

The diet of Calonectris borealis is opportunistic yet specialized, shifting with the seasons and the specific oceanographic conditions of the year. They are primarily piscivorous (fish-eating) and teuthophagous (squid-eating).

Diet Composition

Research from the Azores and other colonies reveals a diet that is heavily reliant on small pelagic fish and oceanic squid.

• Fish: The Blue Jack Mackerel (Trachurus picturatus) is often the single most important prey item, accounting for over 50% of the diet in some years. Other key species include Boarfish (Capros aper), Trumpet fish (Macroramphosus sp.), Saury (Scomberesox saurus), and Chub Mackerel (Scomber japonicus).   

• Cephalopods: Squid are a crucial component, particularly species from the families Ommastrephidae (flying squids) and Histioteuthidae. In some years, cephalopods can appear in nearly 97% of stomach samples, although this statistic can be biased because squid beaks remain in the stomach longer than fish bones.   

Foraging Strategy

Cory’s Shearwaters utilize a variety of techniques to capture prey, often dictated by the behavior of the prey itself.

1. Surface Seizing: The bird lands on the water and dips its head or bill to snatch prey from the surface. This is common when feeding on fishery discards or dense baitballs.

2. Plunge Diving: They perform shallow plunge dives from heights of a few meters, using their momentum to breach the surface.

3. Pursuit Diving: Perhaps most impressively, once underwater, they can use their wings to swim down to maximize depth. While most dives are shallow (less than 2 meters), they are capable of reaching depths of up to 18.9 meters (approx. 62 feet) to pursue escaping fish.   

Table 3: Foraging Metrics and Diet Composition

The table below summarizes key foraging data, highlighting the physiological limits and dietary preferences of the species.

  

Breeding

The life cycle of the Cory’s Shearwater is defined by high investment in a single offspring and extreme site fidelity. These are long-lived birds, with individuals recorded living over 40 years, returning to the exact same burrow year after year.   

Phenology and the “Honeymoon”

The breeding season is synchronized and protracted, occupying the birds for nearly eight months of the year.

• Arrival (February – March): Adult birds return to the colonies from the southern hemisphere. They spend weeks re-establishing pair bonds and cleaning out burrows.   

• Pre-laying Exodus (April): Before the egg is laid, females depart the colony for a “honeymoon” period at sea. This is not for leisure; it is a critical nutritional phase where they forage intensively to build up the energy reserves required for egg formation.

• Laying (Late May – Early June): The female lays a single, large white egg. There is no replacement clutch if this egg is lost.   

• Incubation (June – July): Incubation lasts 52 to 56 days. Both parents share duties in long shifts (stints) lasting several days to over a week. While one parent sits on the egg, fasting, the other forages hundreds of kilometers away.   

Chick Rearing and Dual Foraging

Once the chick hatches in late July, the parents face a logistical challenge: how to feed a growing chick while maintaining their own body condition. They solve this with a dual-foraging strategy.

• Short Trips: They undertake short trips (1-3 days) to nearby waters to gather food specifically for the chick.

• Long Trips: They alternate these with long trips (up to 20 days) to distant, highly productive waters (such as the African coast) to feed themselves and restore their own body mass. This alternation ensures neither the parent nor the chick starves.   

Table 4: Annual Breeding Cycle

   

Threats

Despite being a numerous species, Calonectris borealis faces a multitude of anthropogenic threats that impact both breeding success and adult survival.

Light Pollution and “Fallout”

The most visible and heartbreaking threat occurs during the fledging season (October/November). Young shearwaters use the natural horizon and starlight to navigate from their burrows to the sea. However, artificial lights from coastal roads, stadiums, and hotels can override this instinct, disorienting the fledglings. They circle the lights until they crash-land on roads or buildings. Once grounded, their anatomy—legs set far back for swimming—makes it nearly impossible for them to take off from flat ground, leaving them vulnerable to vehicles and predators. In the Azores, the SOS Cagarro campaign is a massive citizen-science initiative addressing this. In 2024 alone, the campaign rescued 6,388 grounded birds, providing veterinary care to dozens. Since its inception in 1995, this campaign has saved tens of thousands of birds, becoming a model for seabird conservation globally.   

Fisheries Bycatch

At sea, the greatest threat is interaction with longline fisheries. Cory’s Shearwaters are aggressive scavengers and will dive for baited hooks during line setting.

• Longlines: In the US Atlantic pelagic longline fishery, Cory’s Shearwaters are frequently caught, particularly in the Northeast Coast and Mid-Atlantic Bight regions. Observer data (1992-2012) estimated that thousands of birds are killed by the fleet over time, with bycatch rates peaking in summer and fall.   

• Mitigation: Techniques such as “tori lines” (streamer lines to scare birds), night setting (when birds forage less), and weighted lines (to sink bait quickly) have been proven to reduce mortality significantly. The use of these measures is now often mandatory or highly encouraged in high-risk zones.   

Plastic Pollution

As surface feeders, Cory’s Shearwaters ingest floating plastic debris, mistaking it for food. This causes physical blockages, false satiety, and exposure to toxic plasticizers. The species has recently been designated as a bioindicator for plastic pollution in the OSPAR region (Wider Atlantic), replacing the Northern Fulmar in southern latitudes where the Fulmar does not occur.   

Table 5: Anthropogenic Mortality Factors and Mitigation

  

Migration

The migration of the Cory’s Shearwater is a feat of endurance and navigational precision. It is a trans-equatorial migrant, traveling tens of thousands of kilometers annually in a great “loop migration” around the Atlantic.

The Route

1. Post-Breeding (November): Upon leaving the colonies, the birds fly south, riding the Canary Current and the Northeast Trade Winds.

2. Crossing the Equator: They cross the ecological barrier of the warm, nutrient-poor equatorial waters rapidly. Tracking data shows they significantly increase nocturnal flight activity during this crossing to minimize time spent in unproductive waters.   

3. Wintering (December – February): The population splits into distinct wintering areas. The majority utilize the Benguela Current tracking the rich upwelling off Namibia and South Africa. A significant portion crosses the Atlantic to the Brazil Current and the Uruguayan shelf, while others utilize the Agulhas Current in the Indian Ocean.   

4. Return Migration (February – March): The return journey is a rapid “sprint.” They follow a northwesterly route, often using the wind systems of the Western Atlantic, passing close to the South American coast before curving back across the North Atlantic to the colonies. This return trip is faster than the outward journey, driven by the biological imperative to secure high-quality nesting burrows.   

Navigation: Smelling the Way Home

How do they find their way across a featureless ocean? Groundbreaking research involving sensory manipulation has revealed that olfaction (smell) plays a critical role. In experiments where birds were displaced 800 km from their colony, those with impaired olfactory nerves were unable to navigate effectively, wandering aimlessly compared to control groups that returned home quickly. This suggests they use an “olfactory map” of the ocean, smelling landscape features like dimethyl sulfide plumes produced by phytoplankton to orient themselves.   

Table 6: Migratory Statistics

  

Conclusion: A Sentinel of the Atlantic

The Cory’s Shearwater is more than just a bird; it is a sentinel of the Atlantic’s health. From the volcanic cliffs of the Azores to the cold upwellings of South Africa, its life cycle weaves together hemispheres, ecosystems, and nations. For the birdwatcher on a pelagic trip off the American coast, the sight of a Cory’s Shearwater shearing the waves is a connection to this vast, oceanic web. Its robust population and adaptability offer hope, but its reliance on specific island habitats and vulnerability to modern threats like plastic and light pollution serve as a constant reminder of the fragility of our shared seas.

Additional Insights & Cultural Significance

The Legend of the “Cagarro”

In the Azores, the Cory’s Shearwater is deeply embedded in local culture. Known affectionately as the “Cagarro,” its weird, human-like wailing calls at night were historically mistaken for the souls of drowned sailors or supernatural entities. Today, the bird is a national symbol of the archipelago. The term “Cagarro” is even used as a colloquial nickname for the inhabitants of Santa Maria island. The “SOS Cagarro” campaign has transformed the bird from a noisy nuisance or food source (historically harvested for oil and meat) into a beloved icon of conservation success.   

Unique Physiological Adaptations

• Salt Glands: Like all tubenoses, Cory’s Shearwaters possess supraorbital glands that filter excess salt from their blood, allowing them to drink seawater. The concentrated brine is ejected through the tubular nostrils, often visible as a spray when the bird shakes its head.   

• Olfactory Navigation: Their navigational capability is among the best studied in the avian world. The discovery that they rely on an olfactory landscape rather than just magnetic fields has revolutionized our understanding of how seabirds perceive their environment.