Buller’s Shearwater

Birds Name	Buller's shearwater
Science Name	Ardenna bulleri
Domain	Eukaryota
Kingdom	Animalia
Phylum	Chordata
Class	Aves
Order	Procellariiformes
Family	Procellariidae
Genus	Ardenna
Species	A.bulleri

The vast, open expanse of the Pacific Ocean serves as the stage for one of the most elegant and enigmatic seabirds in the world: the Buller’s Shearwater (Ardenna bulleri). Known to the Māori of New Zealand as the Rako, this species is a master of dynamic soaring, utilizing the wind gradients above the ocean swells to travel thousands of kilometers with minimal energetic expenditure. For the birdwatching and wildlife enthusiast community in the United States, particularly those along the West Coast, the Buller’s Shearwater is a prized autumnal visitor. Its distinct plumage—a striking “M” pattern painted across a dove-grey back—makes it one of the most readily identifiable and aesthetically pleasing tubenoses to grace the California Current.

However, the beauty of the Buller’s Shearwater belies a precarious existence. Unlike many of its cosmopolitan cousins that breed on archipelagos scattered across the globe, the Buller’s Shearwater is an extreme endemic. Its entire reproductive future is tethered to a single island group in northern New Zealand: the Poor Knights Islands. This extreme breeding restriction creates a paradox of abundance and vulnerability. While observers off Monterey Bay or Westport might see hundreds of these birds in a single day, the species remains just one catastrophic event away from extinction.

The ecological narrative of Ardenna bulleri is one of historical resilience, having bounced back from near-extirpation caused by introduced feral pigs in the early 20th century. Yet, modern science has recently uncovered a concerning discrepancy in population estimates. Where once it was believed that millions of these birds roamed the Pacific, rigorous quantitative surveys conducted between 2016 and 2018 suggest the population is a fraction of that figure, necessitating an urgent re-evaluation of its conservation status.

This report aims to provide an exhaustive synthesis of the current knowledge regarding Buller’s Shearwater. It delves into the intricate details of its taxonomy, morphological adaptations for flight, and the specific oceanographic drivers that dictate its trans-equatorial migration. It examines the species’ specialized feeding ecology, its inextricable link with subsurface predators like tuna, and the myriad threats—from plastic pollution to fisheries bycatch—that challenge its survival in the Anthropocene. By integrating historical data with cutting-edge research, we offer a comprehensive profile of a bird that serves as a vital sentinel for the health of the Pacific Ocean.

2. Taxonomy and Systematics

2.1 Classification History and Nomenclature

The Buller’s Shearwater was first scientifically described in 1888 by the eminent English ornithologist Osbert Salvin. The specific epithet bulleri honors Sir Walter Lawry Buller, a dominant figure in 19th-century New Zealand ornithology who provided the type specimen. Historically, the bird has been known by several common names, including the Grey-backed Shearwater and the New Zealand Shearwater, reflecting both its plumage and its endemic breeding status.

For much of the 20th century, the species was placed within the genus Puffinus, a large grouping that included most of the world’s smaller shearwaters (e.g., the Manx Shearwater, Puffinus puffinus). However, advances in molecular phylogenetics have led to a significant restructuring of Procellariiform taxonomy. Genetic analysis revealed a deep evolutionary split between the smaller Puffinus species and a clade of larger-bodied shearwaters. Consequently, Buller’s Shearwater was resurrected into the genus Ardenna, a classification now widely accepted by major ornithological authorities including the International Union for Conservation of Nature (IUCN) and the American Ornithological Society.

2.2 Phylogenetic Affinities

Within the genus Ardenna, A. bulleri belongs to the “black-billed wedge-tailed” group, often referred to as the Thyellodroma superspecies. Its closest living relative is the Wedge-tailed Shearwater (Ardenna pacifica), a ubiquitous tropical seabird. Despite their genetic proximity, the two species have diverged significantly in their ecological niches. A. pacifica is a pan-tropical resident, rarely venturing into cold waters, whereas A. bulleri has evolved as a temperate and subarctic specialist, undertaking massive trans-equatorial migrations to exploit the nutrient-rich waters of the North Pacific.

The evolutionary history of shearwaters in the New Zealand region is profound. Fossil evidence from the Pliocene epoch reveals the presence of extinct relatives such as Ardenna davealleni and Ardenna buchananbrowni. These discoveries indicate that the Ardenna lineage has been a dominant component of the South Pacific avifauna for millions of years, adapting to fluctuating sea levels and shifting climatic regimes. A. bulleri represents the extant survivor of this specific lineage, uniquely adapted to the contemporary configuration of the Pacific Gyres.

2.3 Subspecific Variation

Ardenna bulleri is a monotypic species, meaning no subspecies are currently recognized. This lack of subspecific variation is a direct reflection of its breeding biology. With gene flow concentrated almost entirely on the Poor Knights Islands, there has been no geographic isolation of breeding populations to drive divergence. The population functions as a single, panmictic unit, mixing freely across the North Pacific during the non-breeding season before converging on the same few hectares of land to reproduce.

3. Description and Identification

3.1 Field Identification

For birders on a pelagic trip off the US West Coast, the Buller’s Shearwater is often the highlight of the day. It is widely regarded as one of the most beautiful shearwaters due to its clean, crisp markings and elegant flight.

• Dorsal Plumage: The bird’s upperparts are a soft, bluish-grey, a color rare among the typically brown-and-black shearwaters. This grey canvas is interrupted by a bold, blackish pattern across the greater coverts and primaries which, when the wings are spread, forms a distinct, wide “M” or “W” shape. This “M” mark is diagnostic and visible even at considerable distances.

• Ventral Plumage: The underparts are a pristine, brilliant white. This stark white extends to the underwings, which are almost entirely white, bordered by narrow dark margins. This creates a flashing effect as the bird banks in flight—grey, then white, then grey—making it highly conspicuous against the dark ocean surface.

• Head and Bill: A sharp black cap covers the crown, extending just below the eye, contrasting vividly with the white cheeks and throat. The bill is long, slender, and blue-grey with a dark tip, adapted for surface seizing rather than deep diving.

• Tail: The tail is relatively long and wedge-shaped, a feature it shares with its tropical cousin, the Wedge-tailed Shearwater, though the Buller’s tail is dark and contrasts with the grey rump.

3.2 Biometrics

Buller’s Shearwater is a medium-to-large shearwater. While it shares the skies with the Sooty Shearwater (Ardenna grisea) and the Pink-footed Shearwater (Ardenna creatopus), it is generally lighter and more gracile in build.

Table 1: Comparative Biometrics of Pacific Shearwater Species

The weight comparison is particularly telling. A Buller’s Shearwater weighs roughly half as much as a Sooty Shearwater, despite having a similar wingspan. This lower mass relative to wing area profoundly influences its flight mechanics.

3.3 Molt and Plumage Cycles

Unlike some seabirds that have distinct breeding and non-breeding plumages, Ardenna shearwaters generally maintain the same appearance year-round. However, the condition of the flight feathers provides clues to their annual cycle. Buller’s Shearwaters undergo a complete molt of their flight feathers primarily during the non-breeding season in the North Pacific. Birds observed off California in late summer often show signs of active wing molt or fresh plumage, contrasting with the worn feathers of birds just arriving from the southern migration.

4. Morphology, Aerodynamics, and Physiology

4.1 Flight Mechanics: The Art of Dynamic Soaring

The flight of the Buller’s Shearwater is distinctively buoyant and graceful, often described as more effortless than that of related species. While the Sooty Shearwater engages in rapid, stiff-winged flapping, the Buller’s Shearwater relies heavily on gliding and arcing.

This flight style is a function of wing loading (the ratio of body mass to wing area). Birds with high wing loading, like the Sooty Shearwater, must fly fast to generate lift and often flap frequently. They are built like fighter jets—compact and powerful, adapted for underwater propulsion as well as flight. In contrast, the Buller’s Shearwater has a lower wing loading. Its lighter body (approx. 400g) supported by broad wings allows it to generate lift at lower speeds and utilize lighter winds.

• Aspect Ratio: A. bulleri possesses high-aspect-ratio wings (long and narrow), which reduces drag induced by wingtip vortices. This is a standard adaptation for pelagic seabirds, maximizing efficiency during long-distance travel.

• Dynamic Soaring: Like albatrosses, Buller’s Shearwaters practice dynamic soaring. They exploit the gradient in wind speed above the ocean waves—climbing into the wind to gain potential energy and descending downwind to gain kinetic energy. Their lighter build allows them to perform these maneuvers with high, sweeping arcs, often banking past the vertical (90 degrees) to reveal their gleaming white underbellies.

Table 2: Comparative Aerodynamic and Diving Metrics

*Note: While exact depth gauge data for Buller’s is scarce compared to Sooties, their diet and anatomy suggest they are surface specialists, similar to Pink-footed Shearwaters, rather than deep divers.

4.2 Sensory Adaptations: The Olfactory Landscape

One of the most remarkable features of Procellariiformes is their sense of smell. The tube-like nostrils (naricorns) atop their bills are not just for salt excretion; they house a highly developed olfactory system.

• Olfactory Bulbs: The ratio of olfactory bulb size to total brain size in tubenoses is among the largest of any bird order.

• Foraging Cues: Buller’s Shearwaters navigate a “smellscape” over the ocean. They can detect dimethyl sulfide (DMS), a chemical compound released when phytoplankton are grazed upon by zooplankton (like krill). By tracking DMS plumes upwind, Buller’s Shearwaters can locate productive foraging grounds from tens of kilometers away, long before the prey is visible. This adaptation is crucial for locating patchy food resources in the vast, featureless Pacific.

4.3 Salt Glands and Osmoregulation

Living a pelagic life means drinking seawater. To manage this high salt intake, Buller’s Shearwaters possess supraorbital salt glands located in grooves in the skull above the eyes. These glands function as “extra-renal kidneys,” extracting sodium and chloride ions from the bloodstream and concentrating them into a fluid that is far saltier than seawater. This brine is then excreted through the tubular nostrils and often sneezed out by the bird, a common behavior seen in resting rafts.

5. Distribution and Migration

5.1 The Breeding Epicenter: Poor Knights Islands

The breeding distribution of Ardenna bulleri is arguably its most critical conservation vulnerability. The species nests almost exclusively on the Poor Knights Islands (specifically the two main islands, Tawhiti Rahi and Aorangi) and the adjacent Simmonds Islands. Located 22 km off the east coast of Northland, New Zealand, these volcanic remnants offer the predator-free, forested slopes necessary for burrowing.

While prospecting birds or small numbers have been reported on the Three Kings Islands or Mercury Islands, the Poor Knights host effectively 100% of the breeding population. This creates a “basket and eggs” scenario where a single localized catastrophe—such as a rat introduction or a severe storm—could imperil the entire species.

5.2 The Great Trans-Pacific Migration

Following the breeding season, Buller’s Shearwaters undertake one of the classic trans-equatorial migrations of the animal kingdom.

• Departure (April–May): Adults and newly fledged chicks leave the New Zealand colonies in late autumn (May). They move rapidly northwards, crossing the tropics.

• The Western Route: Evidence suggests a northward migration path through the western Pacific, passing the waters east of Japan. This contrasts with the “figure-eight” pattern of Sooty Shearwaters, which often sweep up the western Pacific and down the eastern Pacific.

• North Pacific Staging (June–August): By June, the birds arrive in the North Pacific Transition Zone (NPTZ). They forage extensively in the boundary waters between the warm Kuroshio Current and the cold Oyashio Current, gradually moving eastward toward the Gulf of Alaska.

• The California Current (August–October): As the boreal summer wanes, the population shifts southeast toward the coast of North America. From August to October, they become abundant in the California Current System, exploiting the rich upwelling zones off Washington, Oregon, and California.

• Return Journey (October–November): The return to New Zealand is rapid, crossing the central Pacific to arrive at the breeding grounds by late October/November for the austral spring.

5.3 Birdwatching Hotspots on the US West Coast

For US birders, the window to see this species is narrow but productive. The birds are most strictly pelagic, rarely coming within sight of land unless deep water approaches the coast or strong winds force them inshore.

Table 3: Top US West Coast Hotspots for Buller’s Shearwater

eBird Frequency Data: Data from eBird and historical surveys confirms that September and October are the months of highest frequency. In exceptional years (often preceding warm water events), thousands may be seen, while in other years, they may be scarce.

6. Population Dynamics: The Missing Millions?

6.1 The “Millions” Myth vs. Modern Data

For decades, ornithological literature cited the global population of Buller’s Shearwater at approximately 2.5 million birds. This figure, originating from estimates in the 1980s by researchers like Harper, has been the standard reference. However, recent science has shattered this assumption.

Between 2016 and 2018, the first-ever comprehensive, quantitative survey of the Poor Knights Islands was conducted. Using reproducible methods such as randomized plots and burrow-scopes, researchers arrived at a drastically different number.

• Current Estimate: 78,645 active breeding burrows (95% CI: 67,176–89,178).

• Total Population: Even when accounting for non-breeding birds (which might double or triple the breeding number), the total population is likely in the hundreds of thousands, not millions.

6.2 Interpreting the Discrepancy

Did the population crash, or was it never that big?

1. Overestimation: It is highly probable that the historical estimate of 2.5 million was an exaggeration based on visual impressions of dense rafts at sea or extrapolations from non-representative high-density burrow areas.

2. Stability: Despite the lower number, the 2016-2018 survey found that the population is not at carrying capacity; there are extensive areas of suitable habitat on the islands that remain unoccupied. This suggests the population might still be recovering or stabilized at a natural level lower than previously thought.

6.3 Historical Recovery

The population trajectory of A. bulleri is a classic island restoration success story.

• Pre-1936: Feral pigs introduced to Aorangi Island dug up burrows and ate eggs and chicks. By 1938, the population on Aorangi had collapsed to an estimated 100 pairs.

• Post-1936: Following the eradication of pigs, the colony exploded. By 1981, estimates suggested a recovery to ~200,000 pairs on Aorangi alone (though we now know this might have been optimistic).

• Present: The islands are pest-free, and the population is secure from terrestrial predators, though marine threats remain.

Table 4: Timeline of Population Estimates

7. Feeding Ecology and Diet

7.1 North Pacific Diet (Non-Breeding)

When in the North Pacific, Buller’s Shearwaters are specialized surface feeders. Their diet is heavily dominated by a single species: the Pacific Saury (Cololabis saira).

• Pacific Saury: This epipelagic fish makes up 71% of the prey mass in birds sampled in the central North Pacific. Saury are surface-schooling fish, making them ideal prey for a bird that does not dive deep.

• Other Prey: They also consume lanternfish (Myctophidae), which migrate to the surface at night, and squid such as the Neon Flying Squid (Ommastrephes bartrami).

• Invertebrates: Crustaceans like krill (Euphausiids) and barnacles (Lepas spp.) attached to floating debris are also eaten.

Table 5: Diet Composition in the North Pacific (Mass %)

Prey Item	Species Example	% Mass	Notes
Fish	Pacific Saury (Cololabis saira)	71%	Dominant prey item. Surface schooler.
Fish	Pacific Pomfret (Brama japonica)	~10%	Likely scavenged (shredded tissue found).
Squid	Neon Flying Squid (O. bartrami)	Trace	Scavenged or juveniles taken at surface.
Crustaceans	Barnacles (Lepas), Krill	Minor	Opportunistic surface grazing.

Source: Gould et al. (1998)

7.2 New Zealand Diet (Breeding)

In New Zealand waters, the diet shifts to exploit local productivity.

• Krill and Salps: They feed heavily on the euphausiid Nyctiphanes australis. Interestingly, they also consume gelatinous zooplankton like salps, which are abundant but nutritionally poor and often overlooked in diet studies because they digest rapidly.

• Fish: Small schooling fish such as Jack Mackerel and juvenile Trevally are key components.

7.3 The Tuna Connection

A critical aspect of A. bulleri ecology in New Zealand is its commensal relationship with subsurface predators.

• The Drivers: Skipjack Tuna (Katsuwonus pelamis) and Trevally (Pseudocaranyx dentex) drive schools of krill and small fish to the surface, trapping them against the air-water interface.

• The Behavior: Buller’s Shearwaters rely on these “work-ups.” They follow tuna schools at high speed, using a “leap-frogging” movement to stay ahead of the feeding frenzy. This association is so tight that recreational fishers often look for “Rako” rafts to locate tuna schools.

• Vulnerability: This dependence means that the overfishing of tuna stocks could indirectly starve the shearwaters by removing the mechanism that makes their prey available.

8. Breeding Biology

8.1 The Breeding Cycle

Buller’s Shearwaters are K-selected species: long-lived, slow-reproducing, and investing heavily in a single offspring per year.

Table 6: Breeding Phenology Timeline

8.2 Nesting Ecology

The birds excavate burrows in the soft volcanic soil under the canopy of Pohutukawa forests. Burrows can be up to 3 meters long. The species is aggressive and dominant on the islands, often displacing smaller species like Fairy Prions from optimal nesting sites.

8.3 Breeding Success

Recent surveys established a baseline for breeding success. In the 2016-2018 study, burrow occupancy was found to be 51.7% on Tawhiti Rahi. This baseline is crucial for detecting future declines due to climate change or food stress.

9. Threats and Conservation

Buller’s Shearwater is classified as Vulnerable by the IUCN. This classification is primarily driven by its restricted range—breeding is confined to one small location.

9.1 Plastic Pollution: A Toxic Diet

Plastic ingestion is a catastrophic threat to shearwaters. While the Flesh-footed Shearwater is the “poster child” for this issue (with 90% of fledglings containing plastic), Buller’s Shearwaters are also highly susceptible due to their surface-feeding habits.

• The Mechanism: Floating plastic fragments accumulate in the same convergence zones (eddies and fronts) where shearwaters forage. The birds mistake plastic for prey or consume it accidentally while seizing surface food.

• Impact: Ingested plastic causes physical blockage, perforation of the gut, and malnutrition (false satiation). Furthermore, plastics leach toxic chemicals (phthalates, PCBs) into the bird’s bloodstream, causing long-term physiological damage.

Table 7: Plastic Ingestion Comparison (Select Shearwaters)

9.2 Fisheries Interactions

• Driftnets: In the late 20th century, the Asian high-seas squid driftnet fishery was a slaughterhouse for seabirds. In 1990 alone, an estimated 5,881 Buller’s Shearwaters drowned in these nets. The UN moratorium on driftnets in 1992 was a major victory for the species.

• Longlines: Today, the risk comes from longline fisheries. Birds dive on baited hooks during the setting process. While A. bulleri is not a deep diver (unlike the White-chinned Petrel), it is aggressive at the surface and vulnerable to hooking. However, recent risk assessments place it in a lower risk category compared to albatrosses.

9.3 Biosecurity and Climate Change

• Biosecurity: The Poor Knights are predator-free. The introduction of Norway rats or stoats would be an extinction-level event for the colony. Strict quarantine measures for visitors are vital.

• Climate Change: As a trans-Pacific migrant, A. bulleri is sensitive to basin-wide shifts. ENSO (El Niño) events suppress upwelling in the California Current, reducing food availability. During the 1982-83 super El Niño, seabird reproduction failed across the Pacific. Warming oceans could permanently shift prey distributions, forcing shearwaters to travel further or face starvation.

10. Cultural Significance

10.1 Māori Tradition

To the Ngātiwai iwi (tribe), who hold mana whenua (territorial authority) over the Poor Knights, the Buller’s Shearwater is a taonga (treasure) known as Rako. Historically, the Rako was a source of sustenance. The young chicks, rich in fat (“muttonbirds”), were harvested from their burrows in late summer. This harvest was managed through rahui (restrictions) to ensure sustainability.

10.2 The Massacre and the Tapu

The human history of the Poor Knights is marked by tragedy. In the 1820s (c. 1823), a war party from a Hokianga tribe attacked the islands while the Ngātiwai warriors were away. The ensuing massacre wiped out nearly the entire population. Following this event, the islands were declared tapu (sacred/forbidden) by the survivors. No one lived there again. This strict tapu, respected for nearly two centuries, inadvertently created the sanctuary that allowed the forest—and the Rako—to thrive undisturbed by human development.

11. Conclusion

The Buller’s Shearwater is a bird of dualities: abundant yet vulnerable, widely traveled yet geographically anchored. It is a biological link connecting the krill swarms of New Zealand to the anchovy schools of California. The collapse of the historical “millions” estimate to a reality of ~78,000 breeding pairs is a sober reminder that we cannot take the abundance of nature for granted.

For the observer on a rolling boat off the coast of Oregon, catching a glimpse of that striking “M” pattern is a privilege. It is a sighting of a survivor—a bird that has withstood introduced predators and industrial fishing. Its future now depends on our ability to keep its breeding islands pest-free, its oceans plastic-free, and its food webs intact. The Buller’s Shearwater is not just a tick on a list; it is a barometer for the health of the entire Pacific Basin.