Bird of the Month: Sooty Shearwater
Written by Andy McCormick
Andy McCormick, 2024 Audubon Washington Helen Engle Volunteer of the Year
Sooty Shearwater is the most common shearwater off the Pacific Coast of Washington from August to October, where flocks in the hundreds of thousands move north from south of the equator after the breeding period.
Sooty Shearwater is a dark seabird that spends most of its life on open ocean. Its plumage ranges from sooty-brown to dark gray and its dark bill is thicker at the base and tip giving it a squeezed appearance. An importance feature of this shearwater that helps with identification at sea are the silvery panels found on the underside of its wings.
The Sooty is similar in color, size, and shape with Short-tailed Shearwater (A. tenuirostris) but the Short-tailed has slightly smaller white panels on the underwing. However, in late summer and fall Sooty Shearwaters outnumber the Short-tails and observers trying to identify these shearwaters should consider a candidate as a Sooty Shearwater first unless proven otherwise (Alderfer).
Sooty Shearwater
Scientific Name: Ardenna grisea
Length: 17.5”
Wingspan: 40”
Weight: 2 lb (900 g)
AOU Alpha Code: SOSH
DYNAMIC SOARING
Sooty Shearwaters have long, narrow, pointed wings which are kept stiff in a flight pattern called dynamic soaring. They use this type of gliding flight when moving over the crests of waves on the updrafts created by the wind at sea. Once over the crest the shearwaters glide into the trough of the wave skimming the water in fast flight to the next crest. During high winds Sooty Shearwaters use the thrust to fly high into the air in an arcing flight before making a plunge dive as they return close to the surface of the ocean ready for the next soaring flight. Photos, videos, and vocalizations of the Sooty Shearwater can be found at the Macaulay Library.
SOUTHERN HEMISPHERIC BREEDER
Sooty Shearwaters breed in large colonies on islands in the southern hemisphere ranging from the Falkland Islands, around Tierra del Fuego, to many islands southeast of New Zealand and Australia. The birds dig nesting burrows which can be up to ten feet in length. One white egg per season is tended by both parents with incubation lasting seven to eight weeks. Once hatched, the young are fed by both parents during night feeding visits. As the chicks grow larger, the visits taper off. Eventually, the feeding stops and the young fledge themselves by going to the sea after a total of 97 days from hatching.
Following the breeding season which lasts from September to May in the Austral summer, Sooty Shearwaters disperse north across the equator in a general northwesterly direction on both the Atlantic and Pacific Oceans. During these trans-equatorial flights Sooties spend most of their time gathered in larger flocks or flying along the edge of the continental shelf moving to cooler waters as they go, feeding on fish, crustaceans, squid, and jellyfish.
POPULATION AND CONSERVATION
Sooty Shearwaters are abundant and widespread and have a world population in the range of 20,000,000 individuals with about 4,000,000 pairs in New Zealand alone (Carboneras, et al). Despite this huge population, there are some indications of decline. Recent research has found decreases in the total number of Sooty Shearwaters at island breeding sites in the New Zealand and Australian region. Over the past 20 years fewer birds have been seen off the Pacific Coast of the United States. Studies suggest that climate change and overall warming of the oceans may be a reason these birds have moved away from the coast and toward cooler waters in the central Pacific Ocean (Carboneras, et al).
The Sooty Shearwater is one a group of similar shearwaters that are considered muttonbirds. Their chicks are harvested for food, and to make oil and soap. The estimated annual harvest of 250,000 chicks by the Māori people in southern New Zealand has long been considered sustainable but is now receiving more scrutiny as some breeding locations are producing fewer birds. Recovery of losses takes a long time for many species of shearwaters. Sooty Shearwaters are among them. Once they leave the breeding range for the first time, young Sooty Shearwaters remain at sea for two to four years. Even if they return to the breeding grounds at that time, most will not begin breeding until they are five to nine years of age, thus continuing their traditional pattern of delayed breeding and slow recovery.
A NOTE ON TAXONOMY
The Latin scientific species name for Sooty Shearwater is grisea which refers to the bird’s gray color. It is assigned to the genus Ardenna. This name was introduced in 1853 by Ludwig Reichenbach, who cited an Italian naturalist named Ulisse Aldrovandi who used a form of the word “ardenna” for seabird. There is currently an international effort to create monophyletic genera (which means that a genus will include only birds that form a single clade made up of a group of organisms which have the same evolutionary ancestor). To accomplish this, scientists have used mitochondrial DNA analysis to review the genetic history of birds. The genus Ardenna was re-introduced in 2022 to include seven similar shearwaters. In addition to Sooty Shearwater, Ardenna also includes Wedge-tailed Shearwater (A. pacifica), Buller’s Shearwater (A. bulleri), Short-tailed Shearwater (A. tenuirostris), Pink-footed Shearwater (A. creatopus), Flesh-footed Shearwater (A. carneipes), and Great Shearwater (A. gravis) (Wikipedia).
Photo credit: Robin Gwen Agarwal | Flickr CC
References available upon request from amccormick@eastsideaudubon.org.