Hold on, you’re saying to yourself, I think I’ve read this post before… You’re not wrong! There are TWO World Migratory Bird Days every year, because – surprise! – migration is a round trip. At least, that’s how we usually define it. There are actually three types of migration, only one of which is round-trip, and if you’re interested, I offer a STEM presentation on this topic – contact me to learn more!
But back to the birds. 🙂
In May, arctic terns were just starting to arrive in their Arctic breeding grounds. Right now, they are headed for Antarctica. If you missed reading about why terns are awesome in the spring, now’s your chance to catch up on these cool birds and their incredible adaptation.
In 2014, Tech Support and I were lucky enough to visit Iceland during June, when migratory birds were nesting. We saw puffins and petrels and gulls and guillemots packed onto jagged cliff edges. But what I remember most is walking through the field back to the tour bus and getting dive-bombed by arctic terns who’d built their nests in the long grass flanking the path.
Today is World Migratory Bird Day, making it the perfect day to talk about terns – the champion migrators of the biological world. Adult terns weigh about 100 g, and every year, they make round-trip journeys of up to 50,000 km. 50,000 km, at speeds of up to 670 km per day.
The “legs” of a tern’s journey are typically different lengths, but for the ease of math, let’s assume they are the same. That’s 25,000 km one-way. The record one-way migration for an adult male humpback whale is 18,840 km.
Terns can live more than 30 years, meaning one tern might travel more than 2.4 million kilometers during its lifetime. That’s the equivalent of about three trips to the Moon and back. I’m exhausted just thinking about it!
What could possibly motivate such teensy birds to make such epic journeys? Well, terns nest in the Arctic, during summer in the Northern Hemisphere: they arrive at their breeding grounds in May.
Terns molt – shedding old feathers and growing new ones – in Antarctica, during summer in the Southern Hemisphere. In other words, terns never experience winter: they are always in the right place, at the right time, to take advantage of long summer days and mild temperatures.
Migration timing also seems to be linked to periods of peak ocean productivity. Scientists tracking terns with tiny GPS loggers have shown that terns arrive in each staging location right when the most fish and krill are available to eat – important, because those rich food sources give them the energy they need to keep going. When they reach Antarctica, they also tend to hang out on icebergs or the edge of the pack ice. This gives them solid surfaces to rest on, but really easy access to open water for fishing in.
For terns, the benefits of their extreme migrations seem clear. But migration is not without its risks – for terns and many other species. All that travel takes a ton of energy. Birds can get lost or blown off course in bad weather. They have to pass through areas of unsuitable habitat – places without food, or places inhabited with unfamiliar predators. Birds that orient using natural light cues often get confused by the lights of human cities.
For some ways you can help protect birds this World Migratory Bird Day, check out these lists:
Arctic terns are one of several migratory birds that appear in Polar: Wildlife at the Ends of the Earth. The book is out now – grab your copy at your favourite indie bookseller, or one of the online retailers below:
I’m now booking Polar-themed school and library visits! Contact me to inquire about dates and rates.
Calling all educators! I’m presenting at the North American Association for Environmental Education conference later this month, with fellow children’s science author Rochelle Strauss. The event is virtual, registration is open, and we’d love to see you there!