The Arctic Tern (Sterna paradisaea) is a remarkable seabird known for undertaking the longest migratory journey of any animal on Earth. Each year, these elegant birds travel from their Arctic breeding grounds to the Antarctic and back, experiencing two summers annually. This incredible feat of endurance allows them to witness more daylight than any other creature on the planet. The total distance covered in their annual round-trip migration can range from approximately 44,000 to 59,000 miles (70,900 km to 81,600 km) for some populations.
The Quest for Continuous Daylight and Food
Arctic Terns embark on their extensive pole-to-pole migration primarily to pursue near-constant daylight and the abundant food sources associated with it. This strategy allows them to maximize foraging time and energy intake. By following the sun, these birds ensure a continuous supply of small fish, crustaceans, and marine invertebrates, which are crucial for their survival.
During their migration, Arctic Terns primarily feed on small fish species, such as sand lances, sand eels, herring, and capelin. They also consume marine invertebrates like krill, which provides the necessary energy for their long flights. The birds are adept hunters, plunge-diving into the water to snatch prey from the surface or just below it. They often forage in groups with other seabirds, benefiting from predatory fish that drive smaller fish to the surface. This continuous access to food across hemispheres is fundamental to sustaining their journey.
Ideal Breeding Grounds
The Arctic region offers specific advantages that make it an ideal breeding ground for Arctic Terns. The Arctic summer provides exceptionally long daylight hours, with some areas experiencing 24 hours of continuous daylight. This extended daylight offers ample time for parent birds to forage for food and feed their rapidly growing chicks. The abundance of food sources, particularly fish and invertebrates, is vital for the rapid development of the young during the Arctic summer.
Remote Arctic nesting colonies often provide relative safety and reduced predator pressure compared to more temperate regions. Arctic Terns typically nest on the ground in treeless areas, such as rocky or sandy beaches and small islands. They are colonial nesters, forming large groups that can range from several to tens of thousands of pairs, benefiting from group defense against predators like large gulls. This combination of abundant food, extended daylight for foraging, and relative safety creates an optimal environment for successful reproduction.
Avoiding Arctic Winter Extremes
Arctic Terns leave their northern breeding grounds to escape the harsh conditions of the Arctic winter, compelling them to migrate south. The Arctic winter brings extreme cold, prolonged periods of darkness, and a drastic reduction in food availability. As seas freeze over, prey becomes scarce, making it unsustainable for the terns to remain due to high energy demands and lack of sustenance.
Staying in the Arctic during winter would require significant physiological adaptations and energy reserves that are not feasible for these birds. While some Arctic birds can survive winter by huddling, burrowing in snow, or changing their diet, the Arctic Tern’s primary food source, small fish, becomes unavailable. Therefore, migrating to the Antarctic summer, where conditions are more favorable and food remains plentiful, ensures their survival.