Arctic terns are seabirds known for their long yearly migration from the Arctic to the Antarctic. They breed in the far north during the summer months before embarking on an extraordinary journey southwards in the autumn. Their migration is the longest of any bird species on Earth.
Arctic terns are notoriously aggressive birds when it comes to defending their nests and young chicks. Their fierce nature earned them the nickname “sea swallows” from early Arctic explorers who witnessed their bravery firsthand. Today, Arctic terns continue to show incredible ferocity against any perceived threat.
But why are they so aggressive? The answer lies in their biology and natural breeding behaviors.
Why Are Arctic Terns So Aggressive?
There are several key reasons why Arctic terns will attack anything that comes near their nests during breeding season:
Strong Nesting Instincts
Arctic terns have a very strong natural drive to protect their eggs and chicks. Nesting on the ground makes eggs and young vulnerable to predators like foxes, bears, and gulls. Aggression is an effective defense strategy.
Stress of Migrating Long Distances
Arctic terns undergo an energy-intensive migration each year from the Arctic to Antarctica and back again. The stress of this journey makes them highly sensitive to any disturbances while breeding.
Limited Nesting Habitat
Suitable Arctic tern nesting habitat is scarce. Competition is fierce for the flat, rocky areas that provide camouflage from predators and shelter from harsh weather. They cannot afford to lose their nests.
Ground Nesters
Unlike other seabirds, Arctic terns nest on the ground rather than cliffs. This makes them and their eggs/young much more vulnerable to threats from animals and people on foot.
Deterrent Behavior
Aggression works to scare away potential predators and competitors. Their territorial attacks help secure the space and resources they need to successfully raise young.
When Are They Most Aggressive?
Arctic terns exhibit aggression any time during breeding season from May to early September. However, they reach peak aggression levels at certain sensitive times:
Courtship & Nest Building
Arctic terns return to breeding grounds in spring already pumped with hormones. Males will attack other terns entering their newly established territories. Pairs will dive-bomb any intruders near the nest site they choose.
Incubation
Sitting on eggs makes parent terns feel especially protective. They will readily strike at any animal or person approaching too close during incubation from late May-July.
Hatching & Chick Rearing
The presence of vulnerable chicks brings out maximum aggression in parent terns from July-early September. They use loud shrieking and repeated diving attacks on nest intruders.
Examples of Aggressive Tern Behavior
Here are some firsthand experiences that demonstrate the incredible ferocity of nesting Arctic terns:
- Dive-bombing people’s heads drawing blood
- Hitting intruders with sharp beaks
- Defecating on predators
- Knocking hats off or stealing caps
- Shrieking loudly at any nearby animal or human
- Chasing people hundreds of feet away from nests
Researchers in Antarctica reported 300 aerial strikes per hour from angry terns! This is why many Arctic breeding sites now have posted warnings for humans to wear hardhats.
Defense Behaviors Up Close
A closer look at breeding Arctic terns reveals some of the specific defensive actions they take against intruders:
Chest-Pointing
Terns will point their chests directly at an intruder. This shows they are alert and prepared to attack if the intruder continues approaching.
Circling Overhead
By flying in circles above an intruder, Arctic terns are signaling their perception of a threat and harassing it to leave the area.
Dive-Bombing
From the circling position, terns will swoop down directly at intruders from behind, often striking the person on the head or back before swinging upwards.
Jabbing
At close range, terns jab sharply with their pointed beaks to cause pain and scare away the intruder.
Aerial Screeching
Loud, grating calls are made constantly during an attack. This noisy shrieking serves to attract other terns to mob the intruder.
Why Don’t The Attacks Deter Predators?
Given the incredible aggression shown by Arctic terns, you might assume potential predators would be deterred. However, certain predators are unphased by their attacks:
Thick Fur & Skin
The beaks and claws of Arctic terns don’t do much damage against the thick fur and skin of foxes and bears. The predators ignore their harassment efforts.
Hunger Motivation
If a predator like an Arctic fox is very hungry, it will tolerate tern attacks to grab eggs and chicks from the nest. Food reward outweighs defense risk.
Size Difference
Large predators like polar bears and walruses easily tolerate the small size and limited energy reserves of attacking terns.
Nest Raiding Strategy
Some predators like gulls will raid tern colonies in groups. Multiple large predators will override the dive-bombing defenses of the smaller terns.
Do the Attacks Ever Work?
Though Arctic tern aggression doesn’t deter all predators, their defensive strategy is still often effective at protecting eggs and chicks. Some examples where attacks help:
- Scaring away curious but non-predatory animals like reindeer or birds of other species
- Chasing individual foxes and gulls from the nesting site
- Redirecting charging humans or animals away from nests
- Alerting mate or nearby terns to mob threat as a group
- Causing sufficient pain to humans to make them leave the area quickly
Additionally, their aggression shapes nest site selection over generations. Terns nest on islands and remote peninsulas where large mammal predators are scarce and attacks can better repel threats.
Is Their Aggression Getting Worse?
Anecdotal reports indicate tern aggression at breeding colonies has intensified in recent decades. However, there is no clear scientific consensus yet if attacks are increasing and why.
Research Difficulties
Quantifying changes in tern aggression over long time periods presents logistical challenges:
- Nesting sites are remote and hard to monitor annually
- Individual bird behavior varies each year
- Habitat conditions, weather, predators fluctuate annually
- No established protocols for measuring aggressive intensity
Threats Appear to be Rising
Many experts argue external threats to Arctic terns are increasing, necessitating a stronger defense:
- More human disturbance from recreation, industry, research in the tundra
- Increased predation as polar bears spend more time on shore due to melting sea ice
- Greater competition for scarce nesting habitat as sea levels rise
- Spread of predators like red foxes into tern nesting territory as climate warms
Jury Is Still Out
Despite correlations with increasing threats, concrete evidence of rising tern aggression over recent decades remains lacking. The jury is still out on this question awaiting more extensive, long-term scientific study.
How to Avoid Attacks
If you ever encounter nesting Arctic terns, here are some tips to minimize aggressive attacks from them:
- Wear a hat or helmet for head protection
- Hold an object over your head like a stick or umbrella
- Move quickly through nesting areas
- Visit nesting sites only during non-breeding months
- Follow posted instructions and stay outside designated buffers
- Never provoke, chase, or attack the birds
Conclusion
In summary, Arctic terns definitely deserve their reputation as one of the most aggressive seabirds in the world. Their territorial ferocity reaches a peak during breeding season as they zealously guard their nests and young. While intruders may view their attacks as a nuisance, the terns are simply following strong natural instincts to reproduce and survive as a species. Caution and understanding from humans can help avoid conflict and appreciate these unique migratory seabirds.