The short tailed shearwater (Puffinus tenuirostris) and the Sooty Shearwater (Ardenna grisea) are two similar looking seabird species that belong to the Procellariidae family. Both species are medium-sized shearwaters with long, narrow wings adapted for dynamic soaring flight over the open ocean. They utilize a “shearing” flight technique that takes advantage of updrafts over the ocean waves to travel enormous distances while minimizing energetic costs. While the short tailed and Sooty shearwaters occupy overlapping oceanic habitats and migration pathways, there are some key differences between these species in terms of their appearance, breeding behaviors, foraging ecology, and conservation status.
Physical Appearance
The most noticeable distinguishing feature between the short tailed shearwater and Sooty shearwater is bill morphology. As its name suggests, the short tailed shearwater has a shorter, stouter, blue-gray bill compared to the longer, more slender, dark bill of the Sooty shearwater.
In terms of body size and mass, short tailed shearwaters reach lengths of 38–43 cm and weigh 450–790 g. Sooty shearwaters are slightly larger at 40–51 cm in length and weighing 550–1000 g. Both species exhibit sexual size dimorphism, with males being larger than females on average.
The plumage of short tailed shearwaters is mostly blackish-brown above and white below, with a collar of gray-brown smudging on the breast. Sooty shearwaters have blackish-brown plumage all over, giving them a uniformly dark appearance, aside from a silvery panel in the center of the underwing that is visible in flight.
Breeding Behavior
Both short tailed and Sooty shearwaters nest in large colonies on offshore islands. However, there are some differences in their breeding phenology and behaviors.
Short tailed shearwaters have an elongated breeding season, arriving at colonies in southern Australia in late September and departing in early May after raising a single chick. They dig burrows in sandy soil or utilize crevices in rocks or roots of vegetation.
In contrast, Sooty shearwaters undertake a highly punctual and brief breeding season. They arrive synchronously at colonies in New Zealand in late September, lay a single egg in an open grass-lined scrape, and depart the colony site with their chick in early April, all within a span of 6 weeks.
An interesting behavioral adaptation of short tailed shearwaters is that they routinely fast throughout incubation and provisioning of chicks. Parents can lose over 40% of body mass while rearing offspring. Sooty shearwaters do not fast during breeding and make frequent provisioning trips to deliver whole fish to chicks.
Foraging Ecology
The short tailed shearwater and Sooty shearwater exhibit broadly overlapping foraging behaviors and diets during the nonbreeding season. Both are generalist predators and scavengers, feeding mainly on fish, krill, squid, and other marine invertebrates. They often associate with fishing vessels to exploit discards and processing waste.
However, subtle differences in bill structure influence prey capture techniques. The stout bill of short tailed shearwaters is efficient at surface seizing and diving plummeting down to depths of over 70 m to capture prey. Sooty shearwaters use their more slender bill to snatch prey while hydroplaning across the surface, up to depths of 20 m.
Isotopic analysis shows that short tailed shearwaters have a more diverse, omnivorous, and opportunistic diet compared to Sooty shearwaters during breeding. The longer incubation fasts of short tailed shearwaters are likely linked to their more varied diet.
Migration
Both shearwater species undertake phenomenal transequatorial migrations in the Pacific basin after breeding. Sooty shearwaters hold the record for the longest animal migration, traveling 65,000 km annually between New Zealand and North Pacific foraging areas off Japan and Alaska.
Short tailed shearwaters also migrate enormous distances up to 15,000 km roundtrip between southeastern Australia and North Pacific waters. Their migration pathway forms a figure eight loop that crosses the equator twice in each direction.
Satellite tracking shows Sooty shearwaters utilize more offshore routes concentrated east of New Zealand, while short tailed shearwaters migrate closer to continental landmasses off Australia, Asia, and North America.
Population Trends
The global population of short tailed shearwaters was estimated at 23 million individuals in 2011, with over 80% of the total breeding population concentrated in southern Australia. Population trends appear to be stable with no evidence of significant declines.
In contrast, Sooty shearwater populations have undergone precipitous declines since the 1980s, particularly on South Island, New Zealand. Causes are unclear but likely involve mortality from commercial fishing and other anthropogenic disturbances across the migration range and in Antarctic waters. Sooty shearwaters are currently classified as Near Threatened with an estimated global population of 6.2 million birds.
Key Differences Summary
Feature | Short Tailed Shearwater | Sooty Shearwater |
---|---|---|
Bill morphology | Short, stout, blue-gray bill | Long, slender, dark bill |
Body size | 38-43 cm long, 450-790 g mass | 40-51 cm long, 550-1000 g mass |
Breeding season | Prolonged, late September to early May | Highly punctual, late September to early April |
Nest type | Burrows | Open grass-lined scrapes |
Foraging | More diverse, surface seizing, plunge diving | More specialized, hydroplaning, surface feeding |
Migration | More coastal, figure eight loop | More pelagic, elliptical |
Population trend | Stable at 23 million | Declining, Near Threatened |
Conclusion
In summary, while short tailed and Sooty shearwaters occupy similar ecological niches and exhibit many common life history characteristics, there are subtle differences between the species in morphology, breeding strategies, foraging techniques, migration routes, and conservation status. Both species make remarkable migrations between breeding sites in the southern hemisphere and foraging grounds in northern latitudes. Ongoing tracking, surveys, and ecological studies of these far-ranging seabirds can provide valuable insight into the health of marine ecosystems across ocean basins. Comparative research helps illuminate the mechanisms underlying adaptation in seabirds to utilize distant resources while maintaining fidelity to natal colonies each year.