Arctic terns are sea birds that breed in the Arctic and subarctic regions. They have one of the longest yearly migrations of any bird, traveling up to 25,000 miles roundtrip each year between their Arctic breeding grounds and their wintering grounds in the Southern Ocean around Antarctica.
Breeding Habitat
Arctic terns breed in coastal and inland areas in the high Arctic, usually on small islands, gravel bars, and spits. Their breeding habitat tends to be near ponds, lakes, rivers, or along the seashore, where there are abundant small fish and marine invertebrates to feed on. Ideal breeding sites have a good mixture of vegetation and open, rocky ground. The vegetation provides nesting material and cover, while the open ground allows easy take off and landing.
Some key characteristics of good arctic tern breeding habitat include:
- Islands and coastal areas free from land predators like foxes or rodents
- Gravel beaches, rocky outcrops, and flat, low-lying land along the seashore or near ponds/lakes
- Abundant small fish and marine invertebrates to feed on
- Low, sparse vegetation for nesting cover
- Open, uncultivated ground with good visibility for detecting predators
Arctic terns avoid forested areas and regions with tall, dense vegetation when breeding. The availability of high quality breeding sites limits their populations in many parts of the Arctic.
Courtship and Pairing
Arctic terns arrive at their Arctic breeding grounds starting in late April or May. When they first arrive, the birds gather in flocks on the breeding grounds and spend several days courting and pairing up. Courting groups, called “dances”, can consist of up to several hundred birds hovering 15-30 meters above the ground.
In the dances, the male terns fly around rapidly flapping their wings while holding fish in their bills. They periodically hover in place and make “karr” calls to attract the attention of females flying nearby. When a female tern approaches a displaying male, they may touch bills briefly before the male offers the fish as a courtship gift.
Courtship feeding helps form pair bonds between males and females. The male’s ability to catch fish demonstrates his fitness as a mate and provider. This courtship phase lasts for several days before pairs disperse over the breeding area to nest.
Nest Building
Once paired up, the male and female arctic tern work together to build a nest scrape on the ground. The nest is simply a shallow depression or hollow in the soil or gravel, sometimes lined with small pebbles, vegetation fragments, or fish bones.
Both sexes gather nesting material, but the male does most of the nest construction while the female shapes the interior. Good nest sites provide some cover from vegetation or rocks while allowing the sitting bird good visibility. Arctic terns rarely use the same nest site in subsequent years.
Egg Laying and Incubation
Egg laying starts around early June, reaching a peak from mid-June to early July. The female lays 1-3 eggs, with 2 being the most common. The eggs are olive to brownish with dark speckling. They are pointed at one end and oval-shaped.
The parents take turns incubating the eggs for around 22-24 days until they hatch. The male and female develop incubation shifts, with one bird guarding the nest while the other is off feeding. They exchange incubation duties every 6-10 days.
Both parents vigorously defend the nest from potential predators. Intruders that venture too close may be dive-bombed and struck with the tern’s beak. Arctic foxes, gulls, jaegers, and ravens sometimes pick off eggs or chicks when the parents are away.
Hatching and Chick Rearing
The chicks hatch out one by one, covered in downy feathers with dark bills and feet. The parents brood them for a week until they are able to regulate their own body temperature. The chicks fledge at around 21-24 days old, but continue to be fed and accompanied by the parents afterwards.
The male and female both collect small fish, marine invertebrates, and other prey to feed the chicks. During the first week after hatching, the chicks are fed tiny items like fish larvae and small crustaceans. As they grow, the parents start to deliver larger fish such as sandeels, sticklebacks, and capelin.
Predation, starvation, and harsh weather cause high chick mortality. Only around 1.5 chicks per pair, on average, survive to successfully fledge each season. After fledging, the parents continue tending the young for another 1-2 weeks as they teach them to fish and fly skillfully.
Migration
Starting in late August or September, arctic terns leave their breeding grounds and embark on their long southern migration. They travel down the coasts of North America and Europe before crossing the Atlantic Ocean to Antarctica. Their total migration route covers over 25,000 miles roundtrip!
The terns fly along coastlines and ice edges where they can rest and feed. They migrate in flocks, traveling mainly during the day and foraging in groups along the way. The birds spend the austral summer feeding in Antarctic and subantarctic waters.
They start the return journey north in March and April. By late May, the terns are back at their Arctic breeding colonies to begin the cycle again. They make this immense migration each year of their life, which can span over 30 years.
Breeding Lifespan and Success
Arctic terns have a very long breeding lifespan compared to other birds. They start breeding at 2-4 years old and continue nesting each year until around 30 years old, although a few exceptional birds have still bred at age 34.
With this long reproductive period, experienced breeders tend to be more successful than younger pairs. Older terns arrive earlier at the breeding sites, establish better territories, and have higher breeding success. Their chicks also tend to have higher survival rates.
Some key factors influencing an arctic tern pair’s breeding success include:
- Age and experience of breeding adults
- Timing of spring arrival at breeding grounds
- Quality and defense of nesting territory
- Frequency of egg/chick predation
- Ample food availability during chick-rearing
- Weather severity during incubation and fledging
In general, the estimated number of chicks successfully fledged per pair ranges from 0.8-2.2 per season. Lifetime reproductive success increases steadily with the parents’ age and experience until late in life.
Population Threats
Arctic terns face a variety of threats on their breeding grounds. As ground-nesting birds, they are vulnerable to predation. Population declines have occurred in areas where predators like arctic fox have been introduced.
Habitat degradation from human disturbance, all-terrain vehicles, drilling operations, and climate change also endanger colonies. Oil spills and chemical pollution can poison the birds and their prey.
Some arctic tern populations in Greenland, Iceland, and Scandinavia have suffered from over-harvest of eggs and adults. Historical hunting for the millinery trade decimated many breeding colonies.
More recently, commercial fishing may reduce food availability for terns in some areas. Entanglement in fishing nets causes many accidental deaths. Despite protections, arctic terns still face an uncertain future in parts of their range.
Breeding Adaptations
Arctic terns possess many unique adaptations that aid their breeding success in the harsh Arctic environment:
- Long migrations – By traveling huge distances, they take advantage of the most productive feeding areas year-round.
- Long lifespan – Their long lives allow terns to gain experience and reproduce for decades.
- Agile flight – They fly gracefully and efficiently for thousands of miles.
- Salt filtration – Specialized glands remove excess salt from their blood and beak.
- Plunge diving – Terns plunge into water to catch fish up to several meters deep.
- Camouflaged eggs – Their speckled eggs blend in with pebbles and vegetation.
- Distraction displays – When a predator approaches the nest, terns pretend to have a broken wing to lure it away.
Terns also aggressively mob predators and intruders as a group to drive them away from their colonies. All these adaptations help maximize their breeding productivity in harsh environmental conditions.
Conclusion
From courtship rituals in the spring to chick-rearing in the summer, arctic terns have adapted specialized behaviors and long migrations to thrive as ground-nesting seabirds in the far north. Their resilience and long reproductive lifespan allow them to breed successfully even in the demanding polar climate. Conservation of their fragile breeding habitats will be crucial for protecting arctic tern populations into the future.