The shearwater is a seabird known for its almost continuous life spent over the open ocean. As a member of the order Procellariiformes, often called “tubenoses” due to their unique bill structure, the shearwater belongs to a group of birds that includes albatrosses and petrels. These medium-sized flyers are classified within the family Procellariidae. Shearwaters are famous for some of the longest trans-equatorial migrations of any bird, traveling thousands of miles each year to follow food resources across the globe.
Defining the Shearwater’s Physical Traits
Shearwaters are characterized by a morphology perfectly tuned for gliding flight over the water’s surface. They possess relatively long, narrow wings that are stiffly held. Their common name comes from the way they fly, appearing to “shear” the water as they glide along wave troughs with minimal flapping. This dynamic soaring technique uses wind currents and wave dynamics to conserve energy during vast oceanic journeys.
The plumage of most shearwater species is generally muted, often displaying a countershading pattern of dark browns, blacks, or grays on the upperparts and contrasting white on the underside. Their bills are slender and hooked at the tip, a structure that helps them grip slippery marine prey like fish and squid.
A defining characteristic of all Procellariiformes is the presence of tubular nostrils. This structure is linked to their highly developed sense of smell, enabling them to detect patchily distributed food sources, such as dimethyl sulfide released by feeding plankton, across wide expanses of ocean. Furthermore, shearwaters have weak legs set far back on the body, which makes them clumsy on land but provides a hydrodynamic advantage when diving or paddling.
Adaptations for a Strictly Oceanic Existence
One of the most significant adaptations is the presence of the nasal salt glands, which are located above the eyes at the base of the bill. Since their kidneys cannot produce urine more concentrated than seawater, these glands provide a mechanism for excreting the excess salt ingested from drinking seawater and consuming marine prey.
The salt glands become active in response to an osmotic load, producing a highly concentrated saline solution. This solution drains through the tubular nostrils and drips off the bill, effectively balancing the bird’s internal water and salt levels.
Shearwaters primarily feed on small fish, squid, and crustaceans. Their foraging involves a combination of surface-seizing and shallow pursuit-diving, often plunging to depths that can exceed 30 meters, where they use their wings for underwater propulsion. Their dense, waterproof plumage helps maintain body temperature and buoyancy during these dives. Additionally, their efficient metabolism and ability to store stomach oil provide a lightweight, high-energy food source for themselves and their chicks, sustaining them during long foraging trips away from the nest.
The Necessity of Migration and Breeding Cycles
The Sooty Shearwater (Ardenna grisea) is one of the most famous examples, performing a trans-equatorial migration that can cover an estimated 64,000 kilometers in a figure-eight loop across the Pacific Ocean each year. This annual journey follows productive feeding grounds from Southern Hemisphere breeding sites to the Northern Hemisphere summer, ranking among the longest animal migrations ever recorded.
The birds exploit the rich coastal currents off places like California, Alaska, and Japan during the Northern Hemisphere summer before returning south to breed. They are pelagic outside of the nesting season, meaning they spend the majority of their lives flying or resting far from any landmass.
When the reproductive season begins, shearwaters return to colonial nesting sites on remote islands or coastal cliffs. They exhibit strong fidelity to their nesting location, often returning to the same burrow year after year. The birds typically nest in burrows dug into the soil or in natural rock crevices.
A single white egg is laid per nesting attempt, and both parents share the duties of incubation and chick-rearing. The parents take turns on long foraging trips, sometimes lasting days or weeks, to gather food for the chick before it fledges and embarks on its own journey to the open sea.