The term muttonbird may initially evoke culinary images rather than the vibrant world of seabirds, but it holds a rich significance in the natural and cultural history of the southern hemisphere. Muttonbirds are several species of medium-sized seabirds belonging primarily to the shearwater family, renowned for their remarkable oceanic migrations, unique breeding behaviors, and cultural importance, especially in Australia and New Zealand. These birds are integral components of marine ecosystems, acting as indicators of ocean health while also maintaining a deep-rooted connection with indigenous traditions.
Scientific Classification
The term “muttonbird” is most commonly applied to a group of seabirds within the family Procellariidae, specifically several species of shearwaters in the genus Ardenna. The two most prominent species referred to as muttonbirds are the Short-tailed Shearwater (Ardenna tenuirostris) and the Sooty Shearwater (Ardenna grisea). Both species are part of the order Procellariiformes, which includes albatrosses, petrels, and fulmars—birds characterized by their tube-shaped nostrils and excellent gliding flight abilities over the ocean.
The Ardenna tenuirostris is native to Australia and is colloquially known as the Australian muttonbird or yolla in Tasmania. Meanwhile, the Ardenna grisea has a broader range, breeding primarily in New Zealand and the South Pacific, and is also known as the New Zealand muttonbird. Both species share similar ecological niches and behaviors, though they differ in size and some physical characteristics.
Geographic Range & Distribution
Muttonbirds are pelagic seabirds, spending most of their lives at sea, far from land. The Short-tailed Shearwater breeds mainly along the southern and southeastern coasts of Australia, including Tasmania, where the largest colonies are found on offshore islands such as the Furneaux Group and the Bass Strait islands. After breeding, these birds undertake an extraordinary migration, traveling as far north as the Bering Sea near Alaska, covering over 15,000 kilometers annually—a remarkable feat for a seabird.
The Sooty Shearwater’s breeding colonies are primarily located on New Zealand’s subantarctic islands, including the Snares Islands, Stewart Island, and the Chatham Islands. This species also exhibits one of the longest known migrations of any bird, migrating from the southern oceans to the northern Pacific, reaching as far as the coasts of California, Japan, and even the Arctic. Their extensive range reflects their status as true oceanic nomads, adept at exploiting marine food resources across vast distances.
Both species spend the non-breeding season roaming the open ocean, often far from their breeding sites, navigating by environmental cues such as ocean currents, wind patterns, and celestial navigation. Their distribution spans temperate to subantarctic waters in the southern hemisphere, making them integral to the biodiversity of these marine regions.
Physical Description
Muttonbirds are medium-sized shearwaters, with the Short-tailed Shearwater measuring around 33 to 38 centimeters in length, boasting a wingspan of approximately 86 to 94 centimeters, and weighing between 350 to 600 grams. The Sooty Shearwater is slightly larger, with a length ranging from 40 to 51 centimeters, a wingspan of 102 to 112 centimeters, and a weight between 450 and 900 grams.
Both species have streamlined bodies optimized for long-distance flight, characterized by long, narrow wings and a somewhat stiff tail. The Short-tailed Shearwater has a brownish upper body and pale underparts, with a distinctive white throat patch, while the Sooty Shearwater is darker overall, exhibiting sooty-gray to blackish plumage with subtle variations. Their bills are slender and hooked at the tip, adapted for grasping slippery prey such as fish and squid.
One notable physical feature common to Procellariiformes, including muttonbirds, is their tubular nostrils, which are used to excrete excess salt from the ocean water they ingest, an adaptation critical for their marine lifestyle. Their legs are short but strong, equipped with webbed feet to assist in swimming and takeoff from water surfaces.
Behavior & Diet
Muttonbirds are expert foragers, spending most of their lives at sea where they feed on a diverse diet of marine organisms. Their primary diet consists of small fish, squid, krill, and other crustaceans. They employ a combination of surface seizing, plunge diving, and shallow underwater pursuit to capture prey, often feeding at night when many marine organisms migrate closer to the surface.
These shearwaters are highly social birds, often seen flying in flocks or resting together on the water between foraging bouts. Their flight is characterized by dynamic soaring and gliding, utilizing wind currents over the ocean surface to conserve energy during long flights. Muttonbirds are also known to follow fishing vessels to scavenge discarded fish and offal, exhibiting opportunistic feeding behavior.
Communication among muttonbirds is primarily vocal, especially in breeding colonies where dense aggregations require distinctive calls to identify mates and chicks. Their calls range from harsh, guttural croaks to softer murmurs, serving functions in territorial defense and parent-chick recognition. Outside the breeding season, they are less vocal and more solitary or loosely grouped at sea. According to Audubon Society, this species is well documented.
Breeding & Reproduction
Muttonbirds breed in dense colonies on remote, predator-free islands, where they excavate burrows in soft soil or nest under dense vegetation. The breeding season occurs during the austral summer, typically from September to April, depending on the species and location. These colonies are often large, with thousands or even millions of birds nesting in close proximity. According to RSPB, this species is well documented.
Both the Short-tailed and Sooty Shearwaters are monogamous during a breeding season, with pairs returning to the same burrow each year. The female lays a single white egg, which is incubated by both parents for about 50 days. After hatching, the chick is fed regurgitated food by both parents over a period of approximately 70 to 90 days until it fledges.
Interestingly, muttonbird chicks undergo a process called “fattening,” where they accumulate significant body fat reserves before fledging. This adaptation prepares them for the long journey ahead as they leave the nest and venture into the open ocean. Following fledging, adults typically leave the breeding grounds and embark on extensive migrations, while juveniles remain at sea to develop survival skills.
Conservation Status
Currently, most muttonbird species are classified as Least Concern by the International Union for Conservation of Nature (IUCN), reflecting their large populations and extensive ranges. The Short-tailed Shearwater is estimated to number in the tens of millions, making it one of the most abundant seabirds in the southern hemisphere. The Sooty Shearwater also maintains a robust population, with millions breeding annually across New Zealand’s subantarctic islands.
Despite their stable status, muttonbirds face threats from habitat disturbance, introduced predators like rats and cats on breeding islands, and changes in marine ecosystems driven by climate change and overfishing. Conservation efforts often focus on protecting breeding sites, eradicating invasive species, and monitoring population trends to ensure these birds continue to thrive.
In some areas, sustainable harvesting of muttonbirds is practiced, particularly by indigenous communities such as the Māori in New Zealand, who have long-standing traditions of harvesting chicks for food. These practices are carefully managed to avoid overexploitation, balancing cultural heritage with conservation.
Interesting Facts
The name “muttonbird” originates from early European settlers, particularly on Norfolk Island and Tasmania, who found the birds’ meat to have a rich, fatty flavor reminiscent of mutton (sheep meat). This culinary association has persisted despite the birds being unrelated to sheep, highlighting the unique relationship between humans and these seabirds.
Muttonbirds are remarkable navigators, undertaking one of the longest known migrations of any bird species. The Short-tailed Shearwater’s annual migration spans roughly 30,000 kilometers round trip, traveling from Australian breeding grounds to feeding areas in the North Pacific and back. This epic journey crosses multiple ocean basins and exposes the birds to varying climatic conditions.
Another fascinating behavior is their nocturnal breeding activity. Muttonbirds typically return to their colonies under the cover of darkness to avoid predation, making their breeding grounds lively with calls and movements at night. This nocturnal habit also helps reduce disturbance from diurnal predators.
Finally, muttonbirds play an essential ecological role by transporting nutrients from the ocean to land through their guano deposits, enriching island soils and supporting unique plant communities. Their presence significantly influences island ecosystems, demonstrating the interconnectedness of marine and terrestrial habitats.
In conclusion, muttonbirds are extraordinary seabirds that embody the dynamic interplay between oceanic life and human culture. Their impressive migrations, specialized adaptations, and cultural significance make them a captivating subject for birdwatchers, researchers, and nature enthusiasts alike. Protecting these oceanic nomads ensures the preservation of vital marine ecosystems and the continuation of traditions that honor their place in the natural world.
