The Sooty Shearwater is a large seabird species that feeds primarily on fish and squid. In this article, we will explore the Sooty Shearwater’s diet and foraging habits in detail.
The Sooty Shearwater (Ardenna grisea) is a medium-large shearwater species that breeds on islands in the Southern Hemisphere. They are highly pelagic birds, spending most of their lives out at sea except during the breeding season. Sooty Shearwaters are known for their impressive trans-equatorial migrations, traveling from their breeding grounds in New Zealand and southern South America all the way to the North Pacific Ocean.
Like other seabird species, the Sooty Shearwater relies on the ocean for sustenance. Their diet is composed primarily of fish, squid, crustaceans, and other marine invertebrates. Sooty Shearwaters employ a range of foraging techniques to catch prey while in flight above the water’s surface. The specific components of their diet vary depending on location, season, and food availability.
Major Prey Species
Research indicates that the bulk of the Sooty Shearwater’s diet is comprised of fish and squid. Important fish prey includes lanternfish, anchovies, sardines, mackerel, and saury. Squid species frequently eaten include muscular flying squids, warty squids, and giant squids measuring over 3 feet long. Crustaceans like krill and amphipods are also taken in some regions.
The table below shows some of the main prey species that make up the Sooty Shearwater’s diet in different parts of their range:
Location | Key Prey Species |
---|---|
Northeastern Pacific Ocean | Northern anchovy, Pacific saury, Pacific sardine, market squid, juvenile rockfish |
Southeastern Pacific Ocean | Peruvian anchovy, Araucanian herring, Chilean jack mackerel,warty squid |
Southwestern Atlantic Ocean | Argentine anchovy, Argentine hake, shortfin squid |
New Zealand | Pilchard, arrow squid, red cod, sprat |
The abundance and availability of these prey species in a given region directly impacts the Sooty Shearwater’s diet composition.
Dietary Variation by Location
Several studies have shown that the specific makeup of the Sooty Shearwater’s diet can vary significantly depending on the location and season. This is because the productivity and composition of marine ecosystems differs across their range.
Northeastern Pacific Ocean
In the northeastern Pacific near Alaska, juvenile gadid fish like Pacific cod make up a large portion of the Sooty Shearwater’s summertime diet. The birds also feed on energy-rich Pacific herring and juvenile pollock. In this region, euphausiids like krill are a key prey item during the fall months.
Southeastern Pacific Ocean
Off the western coast of South America, anchovies and squid dominate the Sooty Shearwater’s diet. Peruvian anchoveta are particularly important, making up over 50% of prey samples in some studies. This reflects the huge abundance of anchovies along the Humboldt Current ecosystem.
Southwestern Atlantic Ocean
In the southwestern Atlantic near Argentina, the Sooty Shearwater feeds primarily on that region’s massive schools of Argentine anchovy. To a lesser degree they take Argentine hake, shortfin squid, shrimp and other invertebrates.
New Zealand
Near New Zealand breeding colonies, the main prey species are pilchard, arrow squid and larval fish. Pilchards are taken in large numbers, particularly during the chick provisioning period from January to May. Squid become more prominent in the diet following the breeding season from June to August.
Dietary Differences by Sex
Some evidence suggests that male and female Sooty Shearwaters may vary slightly in their diets and foraging patterns. A New Zealand study found that female birds tended to consume more arrow squid than males. Males were found to feed on more pilchard and mesopelagic fish species. The sexes also differed in their locations of trophic activity at sea.
These differences may reflect divergent energy requirements between sexes during stressful periods like chick-rearing. They could also relate to size dimorphism, as female Sooty Shearwaters are typically larger than males on average.
Foraging Techniques
Sooty Shearwaters employ a range of foraging techniques to hunt and capture prey while flying over the ocean. Their agility, speed and maneuverability make them highly adept aerial hunters.
Surface Seizing
Sooty Shearwaters often grab prey items directly from the water’s surface. They dip their heads down to snatch exposed fish and squid as they swim close to the surface. This allows them to rapidly seize prey concentrations detected while flying over feeding schools.
Pursuit Plunging
By plunging into the water from flight, Sooty Shearwaters can pursue prey spotted deeper down. They crash dive up to 20 feet deep to seize squid, fish and krill not visible from the air. This energetic technique requires precision flying and skill to successfully catch prey underwater.
Hydroplaning
Sooty Shearwaters have been observed landing on the water and propelling themselves forward by paddling with their feet, while dragging their bills to snare prey. This technique called hydroplaning allows them to scoop up small fish and krill at or just below the surface.
Nocturnal Feeding
The Sooty Shearwater feeds continuously throughout the day and night. Some prey like lanternfish and squid migrate closer to the surface at night. The shearwaters are able to use bioluminescence and moonlight cues to locate and hunt prey after dark.
Daily Food Consumption
The daily food intake of Sooty Shearwaters fluctuates substantially over the course of their annual cycle. During the breeding season, adults consume around 150 grams to over 200 grams per day while provisioning hungry chicks. In the nonbreeding period they eat approximately 75-100 grams per day to meet basic metabolic needs.
Over the course of a year, the average Sooty Shearwater eats an estimated 65 kilograms or 145 pounds of prey. This equates to over 50,000 individual prey items annually per bird on average. Their extreme metabolism and substantial food intake fuels the Sooty Shearwater’s impressive transoceanic migrations and active, aerial lifestyle.
Role in the Food Chain
As prolific predators of fish, squid and other marine life, Sooty Shearwaters play an important ecological role in ocean food chains. Their huge populations exert top-down pressure that shapes marine ecosystems across the Pacific, Atlantic and Southern oceans.
Some key interactions include:
- Regulating populations of small forage fish like anchovies, sardines and pilchards
- Limiting squid abundance in areas like the Humboldt Current region off South America
- Competition with other pelagic seabird species for prey like lanternfish and krill
- Providing food to apex predators like sharks, tuna, fur seals and albatrosses that may scavenge dead chicks or adults
The fortunes of Sooty Shearwaters are thus closely tied to the overall abundance of prey species generated by these vast marine food webs.
Conclusion
In summary, the Sooty Shearwater relies primarily on small pelagic fish and squid to fuel its energetic lifestyle and remarkable migrations. Diet composition shifts based on location and seasonal factors. Foraging is done mostly on the wing using various aerial hunting techniques. As an abundant upper trophic predator, the Sooty Shearwater exerts significant influence over the structure and function of Southern Ocean and Pacific marine ecosystems.