It’s World Migratory Bird Day today!
World Migratory Bird Day is celebrated each year on the second Saturday of May (or October, depending on where you are), and is part of a campaign to raise awareness of migratory birds and the issues that they face.
Migrating birds pose unique challenges in conservation, not least because they require protection and healthy habitats at sites which may lie thousands of miles apart and in countries run by governments with different environmental protections and outlooks. The physical act of migration itself is also immensely demanding.
Interestingly, despite long, taxing journeys, migrant birds do not necessarily expend more energy over their annual life cycle than non-migrants. Migration allows individuals to escape cold, harsh conditions in winter, which removes the need to expend energy to stay warm and/or adapt their feeding behaviour as food becomes scarce. In short, migration is a good survival strategy, which is why it has evolved so many times.
Among the 10,000 or so bird species in the world, about 2000, or 20%, are migratory. Further from the equator, where annual conditions are more changeable, this can be as high as 70%. As a result, twice a year, the skies are filled with migrating birds, and natural variation means that the ‘migration seasons’ don’t have strict start and end points – there is always something on the move.
Some individuals may migrate only a few tens of kilometres, while the longest-distance migrant, the Arctic Tern (Sterna paradisaea), a small, 100g seabird, can travel around 90,000km in one return migration from Arctic to Southern Ocean and back …a pretty unfathomable distance, and certainly one that puts my morning 5k into perspective.
What’s even more extraordinary, this means that in its lifetime an Arctic Tern can fly a distance equivalent to three round trips to the moon - which is a poetic coincidence as a commonly held belief in the seventeenth century was that birds flew to the moon in the winter months. The impressive journey of the Arctic Tern is worth repeating to remind us of the amazing life cycle of some birds.
Of the 2000 species of migrant birds, each of the world’s curlew species features on this list, and they can fly quite impressive distances. The Long-billed Curlew (Numenius americanus) is described as a short to medium-distance migrant, moving between the Californian coast and inland grasslands.
The Far-Eastern Curlew (N. madagascariensis) can clock up to 20,000km on a round-trip migration between its breeding range across northeast Asia and its overwintering areas, which include Australia as well as New Zealand, Japan, the Philippines and Indonesia.
Like many other species of bird, it can make such a long journey by doubling its body weight before setting off, allowing it to rely on its fat reserves for fuel. This is common in migratory birds leading one paper to describing them as ‘obese super athletes’. Some individuals can increase their fat content by more than 10% per day.
Another impressive feat that allows long-distance, non-stop flights is the ability to shrink internal organs to take up less space and weight. The Bar-tailed Godwit (Limosa lapponica) can reduce the size of its liver, kidneys and digestive tract by around a quarter. Conversely, when the birds do eat a lot (‘hyperphagia’), the digestive organs grow, allowing more food to be processed.
Birds’ hearts and chest muscles may also expand, helping to power them in flight. Migration is more than just a physical activity – it involves highly tuned, physiological processes.
The Eurasian Curlew (N. arquata) can be seen all year round in the UK, but those which breed here typically overwinter anywhere between Ireland, Portugal and North Africa. Like us, they are individuals, so some stay at home while others travel abroad.
For those that remain in the UK, the majority move to the coast to flock together and feed on mudflats and estuaries; a few spend the winter months on low, wet grasslands. Meanwhile, the British breeding population is bolstered to 150,000 by winter migrants from Fennoscandia.
Eurasian Curlews exhibit chain migration behaviour, which means that those which breed in the north tend to winter furthest north, and vice versa. Males leave the breeding grounds on their autumn migration after the females because they stay longer to guard the chicks.
Unsuccessful nesters may depart earlier than successful birds, and unfortunately today there are many of them. In theory, this means that the poor nesting success that Curlews are experiencing may be impacting their migration, with more birds departing early to winter sites than previously.
Even today, there are still many knowledge gaps when it comes to bird migration, curlews or otherwise. We still don’t know where some populations migrate to, which makes protecting them difficult. This can be helped by tagging – which can be done with coloured rings and/or the use of trackers such as GPS tags. GPS tagging can even help us understand Curlew nesting success by allowing us to monitor their cryptic breeding behaviour without disturbing them.
Once they reach adulthood, Curlews survive well and live a long time, some reaching their 30s. The main struggle they face is producing fledglings, as we discuss in our Curlew Action blogs and webinars. The physical act of migration is not necessarily a taxing time for them, but along with other migratory birds they depend on finding food-rich and safe stopover sites, and it obviously helps if they are not hunted en route.
We can see the risks migrating birds face tragically played in the fate of the Critically Endangered Spoon-billed Sandpiper (Calidris pygmaea)), the illegal hunting of birds in the Mediterranean, and the approximate one billion night-flying birds a year that collide with windows of buildings in the U.S. Migration is risky the world over.
Migratory birds are amazing (and I would highly recommend ‘A World on the Wing’ by Scott Weidensaul to learn more), but they do face many challenges. Many of these are a result of our behaviours, which means we are key to solving them.
Will you be doing anything to mark World Migratory Bird Day? Tag us @curlewaction on social media to tell us what you get up to if so.