The Arctic tern (Sterna paradisaea) is a seabird species that breeds in the Arctic and subarctic regions of Europe, Asia, and North America. Arctic terns are known for having the longest migration of any bird species. They fly vast distances between their Arctic breeding grounds and their wintering grounds in the Southern Ocean around Antarctica. The round-trip migratory flight from the Arctic to the Antarctic and back again can cover over 80,000 km (49,700 miles) for some individuals. This extreme migratory behavior allows Arctic terns to experience endless daylight during the Arctic summer to take advantage of abundant resources and near-constant daylight for feeding chicks. In the winter, they move south to access the resources of the southern polar oceans.
Breeding grounds
Arctic terns have an extensive breeding range across the circumpolar Arctic. In North America, there are breeding colonies in Alaska, Canada, and Greenland. In Eurasia, major breeding areas include Russia, Scandinavia, and Iceland.
Arctic terns typically nest in treeless areas near water, such as coastal beaches, islands, and inland lakes and river deltas. Nests consist of small depressions or scrapes in the ground, lined with vegetation or other debris. The timing of breeding varies across their range, occurring between late May and early July.
Both male and female Arctic terns participate in selecting the nest site, building the nest, incubating the eggs, and raising the chicks. Clutch size is usually 1-3 eggs. After hatching, the chicks are fed and protected by the parents until they fledge at around 3-4 weeks old.
Migration routes
After the chicks have fledged and become independent, Arctic terns embark on their monumental migration in late summer or early fall. They gather in huge flocks numbering in the thousands along coastlines before making the transoceanic journey south.
Different populations follow varying routes across the globe that eventually lead down to Antarctica and the Southern Ocean. Birds breeding in Siberia and Alaska may cross the northern Pacific Ocean, stopping at sites in East Asia before continuing south via the Philippine Sea.
European and Atlantic birds often stage in the North Sea before heading south via the eastern Atlantic Ocean off the coast of Africa. Some fly down the coast of South America. Migration south takes around 6-8 weeks for most Arctic terns.
Wintering grounds
The ultimate winter destination for all Arctic terns is the waters around Antarctica, mostly between 40-65° south latitude. Here they inhabit both coastal and pelagic zones,gathering in large flocks resting on the water.
They spend the southern summer feeding on fish, krill, and squid over the rich waters of the Southern Ocean. Favorite feeding areas include the Weddell Sea, Ross Sea, Amundsen Sea, and Bellingshausen Sea off West Antarctica.
Some individuals may follow ships and trawlers long distances into the pack ice of the Antarctic continental shelf. The wintering period lasts 4-5 months before they make the return trip north.
Return migration
As the southern summer comes to an end, Arctic terns begin the long trek back to their Arctic breeding colonies in late March and April. This northbound migration may follow slightly different routes but also takes 6-8 weeks.
In some cases, individuals return to the exact same breeding site from previous years. Once again, large feeding flocks gather along coastlines and follow specific flight paths across oceans and continents.
Upon reaching the Arctic in late May or June, they reunite with previous mates and the breeding cycle begins anew. By traveling these immense annual migrations, Arctic terns experience more hours of daylight than any other animal on Earth.
Threats during migration
Arctic terns face a diversity of threats during their Globetrotting migrations between the poles. Changing wind patterns, pollution, lack of stopover habitat, and human disturbance can all affect migrating and wintering terns.
Oil spills are a major danger, especially in narrow convergence zones where terns concentrate in high densities. Disorientation from artificial light pollution can cause deadly collisions. Coastal habitat loss threatens essential stopover sites.
Terns are also vulnerable to predation, weather events, changing prey availability, and other challenges over the course of their marathon migrations. Conservation efforts across continents are needed to protect Arctic terns throughout their full annual cycle.
Tracking migration using geolocators
Year | Study Location | Major Findings |
---|---|---|
2007 | Greenland | Average migration distance of 90,700 km recorded |
2009 | Netherlands | Shortest distance traveled was 49,000 km |
2012 | Alaska | Migration routes across Pacific Ocean mapped |
2016 | United Kingdom | Fidelity to wintering sites from previous years shown |
In recent years, scientists have gained new insight into Arctic tern migration through the use of miniaturized tracking devices called geolocators. These tiny data loggers, attached to leg bands, record light levels that allow researchers to estimate latitude and longitude. Studies deploying geolocators on terns breeding in Greenland, Alaska, Europe, and elsewhere have provided new details about migration routes, timing, and individual variation in flight paths. Tracking data confirms the immense distances traveled to Antarctica and back each year by many individuals. Geolocator studies continue to further understanding of tern migratory patterns and behavior.
Longest migration of any animal
The combination of breeding in the high Arctic each summer, and overwintering off Antarctica in the southern summer, gives Arctic terns the longest migratory journey of any known animal species.
The record lifetime migration for an Arctic tern was documented at around 869,000 km (540,000 miles) back and forth between Arctic breeding grounds and the Antarctic wintering region. This is equivalent to flying to the moon and almost halfway back again.
The extreme marathon migrations of Arctic terns are an incredible feat of endurance, navigation, and survival. Their globetrotting lifestyle provides a superlative example of adaptation to make the most of seasonal resources in both polar ecosystems. As winter darkness engulfs the Artic each year, Arctic terns shift hemispheres to capitalize on the bounty of endless Antarctic daylight.
Evolutionary advantages
From an evolutionary perspective, the development of these extensive migrations enhanced the survival and reproductive success of Arctic terns. Breeding in the Arctic provides:
- Abundant prey to raise chicks
- Near constant daylight for hunting
- Lower parasite loads
- Fewer predators
Wintering in Antarctica offers:
- Rich marine food sources
- Open unfrozen habitat
- Extremely long photoperiod for foraging
Together, these factors maximize chances for terns to successfully reproduce, and feed efficiently, over the full annual cycle. The extreme migration is driven by seasonal shifts between optimal breeding and wintering areas.
Navigation and orientation
Arctic terns are incredibly skilled aerial navigators, able to pinpoint locations across entire oceans year after year. They orient themselves using the sun’s position during the day and stars at night. Their magnetic and celestial compasses keep them on course even during long nonstop flights out of sight of land.
Past experience helps guide terns along established routes and to traditional staging and wintering sites. They likely also use their acute sense of smell to locate specific breeding colonies. The sea surface may provide olfactory cues to help navigate along coastlines. Their navigational systems and mental maps enable remarkable precision over such immense distances.
Metabolic adaptations
The extreme endurance flight of Arctic terns requires special metabolic adaptations to power this energy-intensive lifestyle. Terns have very high metabolic rates in flight and efficient fat-burning ability. They can store fat reserves to fuel long nonstop flights.
Selective muscles like the large pectoralis used in flapping have an abundance of oxidative and transport proteins for aerobic respiration. Terns have the capacity to reduce organ sizes during migration as they burn up fats and proteins. Physiological and biochemical adjustments help terns sustain these unbelievable migrations.
Diet and feeding
Arctic terns have a primarily fish-based diet, but also eat a wide variety of marine invertebrates including crustaceans, mollusks, marine worms, and cephalopods. On the breeding grounds, major prey includes small schooling fish like sandeels, capelin, herring, and sand lance.
During migration and in Antarctica, they consume more oceanic species like polar cod, Antarctic silverfish, krill, and squid. The proportions of different prey species vary based on location and seasonal availability. Terns are aerial plunge divers, hunting by hovering and diving to grab prey near the water surface.
Their migrations take advantage of seasonally shifting prey resources between the Arctic and Antarctic. The extremely long summer daylight hours in both regions provides more feeding opportunities.
Threats and conservation
Arctic terns currently face a variety of threats across their circumpolar range. As surface-nesting seabirds, breeding colonies are vulnerable to predation from foxes, gulls, and other species introduced to tern breeding islands by humans. Other threats on the breeding grounds include habitat degradation, pollution, resource competition, and disturbance from human activity.
During migrations, major hazards include oil spills, plastic ingestion, bycatch in fishing gear, and lack of suitable staging areas. Even Antarctica is not free of human impacts, with threats from pollution, tourism, research activities, and climate change.
International cooperation is required to monitor and protect Arctic terns across oceans and national boundaries. Conservation solutions include preserving migration stopover and wintering sites, reducing offshore threats, and safeguarding critical breeding habitat through predator control, designation of reserves, and managing human use. Sustained efforts are needed to ensure the resilience of these global travelers into the future.
Research priorities
Many aspects of Arctic tern biology and ecology remain poorly understood and warrant further research. Priorities for future study include:
– Filling knowledge gaps about migration routes, timing, and connectivity between breeding and wintering grounds. Expand tracking studies using new miniaturized technologies like nanotags and satellites.
– Investigate mortality factors and survival rates over the full annual cycle, especially during the non-breeding period.
– Study the effects of wind patterns, ocean currents, and other meteorological influences on migration.
– Assess potential impacts of climate change on prey availability and Arctic/Antarctic habitats.
– Use genetics to better delineate different populations and analyze population trends.
– Examine physiology and adaptations that enable extreme endurance flights.
Importance of protecting Arctic terns
Despite the resilience of their extreme migratory life history, Arctic tern populations face increasing threats from human activities across the globe. These long-distance aerial migrants exemplify the interconnectedness of ecosystems across latitudes and hemispheres. Conservation requires international collaboration to protect terns throughout their range.
More research and monitoring are essential to guide evidence-based management strategies. Arctic terns serve as an indicator of the health of both polar marine environments. Safeguarding these global travelers ensures we maintain intact ecosystems and coordinated hemispheric stewardship, even in the planet’s far-flung polar reaches. The epic migrations of Arctic terns should inspire global cooperation to protect the world’s shared natural heritage.
Conclusion
The marathon migrations of Arctic terns are one of the great wonders of the natural world. By traveling up to 80,000 km roundtrip each year between Arctic breeding grounds and the Antarctic oceans, Arctic terns experience more daylight than any other creature on Earth. Their migration takes advantage of the bountiful resources of both polar seas to maximize reproductive success.
Arctic terns navigate with incredible precision back to the same sites year after year, demonstrating spectacular feats of endurance across entire oceans. While threats exist across their global range, increased conservation action informed by further research can ensure Arctic terns continue their extraordinary migrations long into the future. The globe-trotting journey of Arctic terns represents one of nature’s great migrations.