39: Arctic Terns

This week is another animal spotlight, but instead of looking at the furry creatures of the north, we will be looking at birds, or one very impressive bird in particular. This is the Arctic tern, who though it lives part of the year in the Arctic, spends much of its time on the wing travelling to far away places. Here we will be looking at what is involved in the life of an Arctic migrant and how it manages its long distance travelling feat. Enjoy!

Arctic Terns Nesting in Iceland (PIcture: Jakub Fryš/Wikimedia Commons/CC BY-SA 4.0)

Types of Arctic Birds

Birds in the Arctic have two choices, either they can hunker down for a cold harsh winter and hope they have the fluff to survive it, or they can escape to warmer climates during the season which is more pleasant but can take quite a lot of energy. Very few birds pick the first option, hunkering down. The ones that do include things like ptarmigans and snowy owls which weather out the winter on land, and the incredible Atlantic Puffins who sit out in the Atlantic Ocean over winter.

The vast majority of Arctic birds go for the second option, seasonal migration. The summer months are spent in the north enjoying all the berries, shrubs and tasty insects flying around in the forests and snowless tundra. Then as winter approaches the birds fly off to far flung warmer places where they can get enough food. This includes birds like eagles, geese, some wagtails and some sparrows.

File:Puffin Latrabjarg Iceland.jpg
The Atlantic Puffin, another northern bird. Picture: Boaworm/Wikimedia Commons/ CC BY 3.0)

The Arctic Tern lifestyle

One of the migratory birds with an incredibly impressive migratory lifestyle is the Arctic Tern. They are medium sized birds with a wingspan of around 75 cm, so between the size of a robin and a crow. Their diet is mostly made up of small fish like sand eels and capelins, though they do eat some crustaceans like crayfish too. Arctic terns are monogamous and lay 2-3 eggs each year.

What these birds are most famous for is their incredible migrations. They spend summer in the mostly treeless parts of the entire Arctic, including Alaska, Canada, Northern Europe, Russia and Greenland, laying their eggs and raising their chicks. At some point in autumn the bird colony falls completely silent in a behaviour which is called “dread”. Then, all together the birds lift of from their nest and the migration begins. For the next two months these bird fly in a roughly south direction with detours here and there, stopping off in almost all parts of the globe along the way until they react the Antarctic Circle.

It is there that they spend ‘winter’, though it is the summer in the southern hemisphere, fishing in the rich waters surrounding the southernmost continent. These journeys, meandering included, can cover up to 70,900 km (44,100 miles) from the Arctic, though some Arctic terns who nest in the Netherlands have been seen to travel even longer distances, up to around 90,000 km (56,000 miles). That is the longest known migration by any animal on earth. Impressive!

It should be noted that whilst their journey south can take many months, the journey back north can be done in little over one month as prevailing winds help to scoot them along quicker.

Geolocation data from 11 Arctic terns tracked from their breeding colonies on Greenland and Iceland all the way to the Antarctic waters. Green is autumn, red is winter and yellow is spring. Data from Egevang et al 2006, link to the scientific paper at the bottom of this plog

Migration adaptations

So how is it even possible to undertake this type of feat? For one, Arctic terns are well physically adapted to flying long distances because of their low weight. Not only do they not hold much extra chub, but these birds, and in fact all birds, have hollow bones. This reduced weight compared to body size makes long distance flight, and all flight really, much easier.

A second advantage is their behaviour. Arctic terns eat food from the oceans, and their hunting mechanism is to hover above the water, then lunge just below it’s surface to catch food. This, and their ability to catch and eat insects during flight means that it is pretty easy for them to eat on the go, as well as whilst crossing large stretches of water where you or I might struggle to find much to eat.

As they are monogamous, Arctic terns also don’t have to waste lots of energy each year fighting for a mate. They just have to do it once and they’ve got a partner for life. To keep up their energy stocks up, Arctic terns have a very high metabolic rate. This means they have to eat food often, but saves them having to store their energy as heavy fat reserves. They can also sleep whilst flying, saving them from having to stop over somewhere each night if they don’t want to. Finally, Arctic terns have a very aerodynamic shape making their flight as efficient as possible.

How do Arctic terns, and other migratory birds, know where to go? It depends a bit but the main method we know of is using visual cues like the position of the sun, the stars, obvious landmarks and perhaps even the smell of certain areas along their journey. It is also thought that birds can tune into the earth’s magnetic field, sort of like a compass, in order to point themselves in the right direction.

File:Arctic tern (Sterna paradisaea) attacking, Amsterdam island, Svalbard.jpg
An Arctic Tern preparing to defend it’s nest. Picture: AWeith/Wikimedia Commons/ CC BY-SA 4.0)

Luckily Arctic terns aren’t an especially endangered species. They have decent numbers at the moment, though there are concerns if they lose a lot of habitat or food sources, either to climate change or further human development such as Arctic oil fields in the future. Natural predators include things like rats and mink who attack nests, and if a human gets too close, the birds will valiantly defend their offspring by pecking you and then pooping on your head. Sound like an effective deterrent to be fair. Their fiestiness can be pretty impressive, as they have been known to chase off polar bears from accidentally wandering onto their nests.

Despite their relatively safe numbers, these birds like many other species have their populations monitored and studied, which is how we know about their long distance flights. However, due to the remoteness of many of the areas where they stay, they definitely aren’t one of the best understood creatures by scientists. Much about them still remains a mystery. We do know for sure though they are one of the most incredible flyers on the planet! Kudos to you little birds!

There are many different kinds of terns. This picture can help you tell them apart a little.

For more info:

Scientific paper on tracking Arctic tern migrations: Click here

There’s not actually that much more info out there about these amazing birds… so here’s just a nice video of them enjoying the skies!

Music: kongano.com

Title Image: Kristian Pikner/Wikimedia Commons/ CC BY-SA 4.0

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