The Arctic tern (Sterna paradisaea) is a seabird that exhibits some fascinating behaviors and adaptations. Arctic terns nest in the Arctic and subarctic regions in the summer, and migrate to the Antarctic region in the winter. This makes them the bird species with the longest migration, flying over 20,000 miles each year. Arctic terns have a number of interesting behaviors related to their breeding, feeding, and migratory habits.
Breeding Behavior
Arctic terns breed in colonies that can range from just a few dozen to several thousand pairs. They nest on the ground in rocky areas near water. Both the male and female help build the nest by scraping a shallow depression in the ground and lining it with whatever materials are available – things like grasses, moss, and seaweed. The nests are often very close together, with just a foot or two between neighboring nests.
Courtship displays like aerial glides and offerings of fish help pairs form strong bonds. Once bonded, the male and female both share incubation duties. They take turns sitting on the nest and guarding their territory. If an intruder gets too close, they will dive-bomb the threat while calling loudly. This aggressive territorial defense helps protect the eggs.
Arctic terns lay between 1-3 eggs. The eggs are well-camouflaged with dark speckles against light tan or olive shells. Both parents share brooding duties, sitting on the eggs to provide warmth. The eggs hatch after around 22-26 days. The semi-precocial downy chicks are able to move around, but rely heavily on their parents for warmth, protection, and feedings.
Raising the chicks is hard work and requires immense cooperation between the bonded pair. The parents catch small fish, marine invertebrates, and even insects to feed the chicks. They present this food to the chicks by carrying it in their beaks. The chicks reach their full adult size in just 20-25 days after hatching, an extremely rapid growth rate. After another few weeks of flying lessons and honing their fishing skills, the young birds fledge and become independent around age 6-8 weeks.
Foraging Behavior
Fishing is the primary foraging strategy for Arctic terns. Their diet consists mainly of small forage fish, marine invertebrates like shrimp or squid, and some insects. Arctic terns have special adaptations that help them adeptly catch prey.
Their long, pointed wings allow them to hover and dive swiftly to snatch prey from the water’s surface. Dense waterproof plumage keeps them dry and buoyant. Spiky little bills are perfect for snatching slippery fish. And excellent eyesight combined with streamlined bodies lets them spot and quickly pursue prey underwater.
Terns plunge dive from heights of around 10 to 30 feet to catch prey near the surface, penetrating just an inch or two into the water. They may also engage in contact dipping, hovering right over the water to pick up food. Aerial pursuit of flying insects is also common. No matter the exact mode of attack, Arctic terns are graceful, agile fliers and hunters.
Terns mostly hunt alone, but may forage in loose groups that cooperate to drive fish into bait balls for easy picking. By foraging in cold northern seas at high latitudes, Arctic terns face less competition for food resources than related terns at lower latitudes.
Migration
The Arctic tern makes the longest yearly migration of any bird species. They breed in Arctic and subarctic regions, then migrate to Antarctic and subantarctic waters for the winter, often traveling over 20,000 miles round trip each year. Their devotion to these extreme polar habitats earns them the nickname “polar nomads.”
Arctic terns follow somewhat flexible migratory routes based on weather and food availability. However, most populations follow certain core migration patterns. For example, birds that breed in Greenland, Iceland, and Northern Europe first migrate southwest towards the waters off Western Europe and Northwestern Africa. From there, they stage further south along the West African coast before making the long journey all the way across the Atlantic to wintering areas off Antarctica.
Terns that breed in Arctic Russia, Alaska, and Canada first fly south through Pacific waters along Japan and China’s coasts before eventually veering east to reach Antarctica. Their nonstop transoceanic flights of thousands of miles are made possible by their aerial stamina and ability to feed on the wing by dipping down to snatch fish at the surface.
These marathon migrations are timed based on seasonal peaks in food availability – terns want to reach Antarctica just as the waters are teeming with overwintering fish and krill. Heading north in the spring gets them back to the Arctic to take advantage of the burst of productivity and food resources there through the summer breeding season.
Navigation and Orientation
The Arctic tern’s amazing long-distance navigation remains somewhat mysterious. Scientists believe they use a combination of strategies to find their way on migrations that span hemispheres. Like many migratory birds, terns are equipped with an internal magnetic compass that lets them get their bearings based on Earth’s magnetic fields. They likely also use the sun as a compass during daytime flights.
In addition, terns may orient themselves based on landscape cues and through path integration, using an internal “map” and keeping track of the directions and distances they fly. Chemical cues like odor plumes from areas rich in prey may help migrating terns stay on course over oceans. And sea swell patterns and polar winds likely also provide navigational inputs.
Young terns on their first southbound migration may even follow older adults and learn the routes. No matter the exact mechanisms, Arctic terns have an incredible ability to navigate directly between their breeding and wintering grounds year after year.
Wintering Behavior
During the Antarctic winter, Arctic terns face nearly constant daylight. They patrol waters along pack ice margins, roosting on ice floes between foraging bouts. Terns winter as far south as the edge of the Antarctic ice sheet around latitude 66°S, one of the southernmost distributions of any bird.
Their winter diet shifts to include more krill and fewer fish compared to the breeding season. Like in summer, they captures prey by plunge diving from the air or dipping down from a hover. Birds wintering further north may also forage in mixed-species feeding flocks with other seabirds.
Wintering flocks are often segregated based on age, with juveniles and adults roosting separately. This may help reduce competition for food between more experienced adults and young birds foraging on their own for the first time. By spreading out over a wide swath of the Southern Ocean, the terns help ensure sufficient food for all.
Threats and Conservation
Although still abundant, Arctic tern numbers have declined in some regions in recent decades. Threats on the breeding grounds include habitat loss, disturbance of nesting colonies, pollution, and predation from introduced species like rats. More research is needed, but climate change and commercial fishing may also negatively impact terns in both hemispheres.
Global warming is of particular concern for a species that relies on such specialized and vulnerable polar ecosystems. Conservation measures like protecting important breeding sites and reducing disturbances are key. But ultimately, addressing climate change and maintaining healthy marine food webs worldwide will be necessary to ensure a bright future for these champion migrators.
Key Facts
- Species: Sterna paradisaea
- Length: 31-39 cm
- Wingspan: 76-85 cm
- Weight: 86-127 g
- Life span: Up to 30 years
- Diet: Small fish, marine invertebrates, insects
- Range: Breeds in Arctic; winters in Antarctic
- Population Status: Declining in some regions
Interesting Facts
- Makes the longest yearly migration of any bird – up to 44,000 miles roundtrip.
- The Old Norse name “Thorshani” hints at Viking reverence for this bird’s navigational prowess.
- Aggressively defends nesting territories, even attacking humans who get too close.
- Chicks hatch with downy black and white camouflage resembling orca coloring.
- One of only three bird species to regularly visit both polar regions (along with sanderlings and Baird’s sandpipers).
Conclusion
The Arctic tern is a fascinating polar seabird that thrives despite its extreme migrations between the planet’s farthest ends. It fills a valuable ecological niche in both hemispheres and has captivated human observers with its grace, power, and endurance since time immemorial. As sea ice declines and marine food webs are disrupted by climate change, the future of this legendary migrator hangs in the balance. Protecting its breeding and wintering habitats will help ensure the Arctic tern remains a symbol of the wonder, connectivity, and resilience of life for generations to come.