Birds are the world’s great migration specialists.
Whether they are travelling in search of warmer weather, more abundant food, suitable nesting sites or longer daylight hours, many bird species cover extraordinary distances, travelling from one side of the globe to the other and back again.
In an impressive feat of biological engineering these migratory bird species have developed amazing physiological adaptations that allow them to be masters of the migration.
Here at Thornybush Nature Reserve, we are beginning to see some of our summer migrants returning. Wahlberg’s eagles and Purple rollers are arriving in good numbers, and before long the air will be filled with the unmistakable mating call of the Woodland kingfisher, one of the sounds that tells us summer is on its way.
For some of our summer migrants, the Lowveld is a breeding ground. Wahlberg’s eagles, Woodland kingfishers and the much rarer Lesser moorhen take advantage of longer daylight hours and an abundance of food to raise their young.
Others have a different reason for being here. European rollers, Barn swallows and European bee-eaters migrate south not to breed, but to escape the colder northern winter and take advantage of the plentiful food available during a Lowveld summer. South Africa is also home to some resident breeding populations of European bee-eaters.
One of the most remarkable things about migration is the precision with which birds navigate. Some return to the same region, and sometimes even the same nesting site, year after year. So, how do they do it?
Built for the journey
A bird’s body is remarkably well adapted for long-distance flight.
We often hear about how birds bones are hollow, which, yes, makes them light enough to fly, but their respiratory system is perhaps even more impressive. This hollow bone structure creates better distribution of oxygen around the body and allows for high quality breathing at higher altitudes while they migrate.
Before a long migration, many birds build up fat reserves, their most important source of energy for the journey. But once they are airborne, every bit of energy matters.
Wing shape plays an important role. Many birds have wing shapes designed for reducing air resistance and they can often be seen flying in a formation which makes cutting through the air tension less challenging. This makes flying easier and aids in the preservation of energy.
To save weight mid-flight something pretty crazy also happens to their organs! Non- essential internal organs like the digestive tract temporarily shrink, making them lighter and therefore save energy. These organs then quickly regrow upon arrival at their destination once they start feeding normally again.
Their chest muscles are also highly specialised. Packed with mitochondria, the structures responsible for producing energy within cells, and supplied by an extensive network of blood vessels, these muscles are designed to sustain the demands of prolonged flight.
But perhaps the most fascinating question is how birds know where they are going.
A natural compass
Birds appear to use a combination of sensory systems to navigate, including the sun, stars, landmarks, smells and, in many species, the Earth’s magnetic field.
Birds have a specialised light sensitive protein in their eyes called Crytochrome 4 which allows them to see the magnetic field of the earth in the form of waves, allowing them to use the magnetic field like a map.
In addition to seeing the magnetic field some birds have microscopic crystals called magnetites (magnetic minerals) in their beaks, which allows them to feel a magnetic pull relative to the earth’s poles. Meaning they can feel the planets magnetic field with their beak.
And finally, recent research on pigeons shows that they have specialised hair cells in the inner ear which may help them to process magnetic data.
These unbelievable adaptations are allowing the bird to combine multiple sensory inputs to navigate with incredible precision over long distances using the earth’s magnetic field as a guide.
And that is what makes migration so extraordinary.
These birds are not simply flying south for the summer. They are undertaking journeys that can span thousands of kilometres, relying on a combination of physiology, instinct and sophisticated sensory systems that scientists are still working to fully understand.
A welcome return to Thornybush
For us, the arrival of the summer migrants is one of the seasonal highlights of the bush.
So, on your next safari at Thornybush, take a moment to look up.
That bird perched in a tree or passing overhead may have travelled thousands of kilometres to get here. And behind that seemingly effortless flight is one of nature’s most remarkable stories of adaptation, endurance and navigation. Welcome back, summer migrants!
