Longest migration ever – The Arctic Tern

The Arctic tern holds the record for the longest migration ever noted among all species of birds. With its grey and white plumage and a black cap, it is known as the ‘Sea Swallow’, weighing on average just 110g and measuring a mere 35cm in length.

This intriguing bird has a worldwide population of two million individuals. With an average lifespan of over 30 years, the Arctic tern can fly a distance of up to 44,000 miles per year – that’s equivalent to three round trips to the moon!

But how does this wonderful creature know how to do all of this? Let us start at the very beginning.

A journey…hatching

The Arctic tern is seen on land during the breeding season, which begins in May and lasts through to July; their main breeding location is in northern latitudes such as the UK.

The size of each clutch varies between one and three, which incubate for up to 24 days, during which time both sexes share incubation duties. The eggs are grey with speckles of brown and, after the eggs have completed incubation, they then hatch after around three weeks.

Arctic tern egg

An Arctic tern egg.

Both parents share duties of care for their young, whose diets consists mainly of fish; the feeding of the young lasts for approximately one month.

After roughly 24 days of development, the chicks will have fully fledged, providing the necessary maturity for venturing off alone.

How do they know?

How interesting it is to “venture off” alone. For us humans, despite whatever motivations we have behind a specific action or set of actions, we are driven by experience; and this experience informs us of how we go about our daily tasks. Take as an example an empty bottle of milk in the fridge. It must be replaced, but how do you know what to do? Well, over the years you’ve learned to go to the supermarket or local store to buy milk in exchange for some form of currency. This is completely natural, right? Possibly, but a lot of this has to do with the way we learn as we grow, and the pliability of the human brain is quite distinct from many of other animals in the natural world.

The typical Atlantic migratory path of an Arctic tern.

Animals require other clues, which bring us back to the notion of “venturing off”. How does the Arctic tern know when to leave? One suggestion is that migration is related to the circadian rhythm. For most birds, migration is a daylight activity, whereby night is for resting. During the night, specific sets of hormones, such as melatonin, are released, giving the birds signals to sleep. When darkness approaches, and longer days wane, this decreased daylight, and ultimately raised levels of melatonin, provide a clue to the bird that it is time to migrate.

Bound for distance…

Once ready, the Arctic tern bird begins its arduous journey, starting in Northumberland and flying down through Britain to either the African or Brazilian coasts, returning in an ‘S’-shaped path up the middle of the Atlantic Ocean, having flown about 44,000 miles.

Like many migratory birds, Arctic terns have an instinct about where to go, even though they have never traced such a path before. This intriguing feature of the avian world is a question that has been posed throughout history, and recent studies suggest a number of techniques for navigating long migratory journeys. The three key “techniques” are derived from the interpretation and use of the sun, the stars and the Earth’s magnetic field – the Robin, for instance, uses its right eye for detecting magnetic fields for navigation. Some birds simply have the basic DNA for flight, yet learn their flight paths from older birds.

With the Arctic tern being a seabird, it is interesting to note how such a bird can navigate what could be considered a barren and featureless environment. For many seabirds, direction is sought through smell.

After the 44,000 mile flight, the Arctic Tern returns in an ‘S’-shaped manner – where they detour on long distances to discover new feeding grounds, or to avoid danger from bad weather. When the birds are migrating they don’t spend the whole time flying – instead they glide for a large proportion of the time. After setting out, the birds rest in the North Atlantic to feed. The Tern flies on average in a lifetime as far as 3 million km making it the longest migratory bird ever.

On the way back to Northumberland they follow the winds, instinctively returning to their original habitat. By having spent a lifetime in perpetual summer, they will have never felt the full effects of winter.

Imagine that.