Migratory birds embody some of the most intricate evolutionary adaptations in the animal kingdom, without which their journeys would be impossible. Here’s how the Pfeilstorch taught us this.
By Zoologische Sammlung der Universität Rostock – Zoologische Sammlung der Universität Rostock per Ticket:2018022810005245, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=66997013
In the small village of Klütz, Germany, in 1822, a stork landed carrying the answer to a question that had been plaguing naturalists for millennia.
The Germans who spotted the bird quickly discovered that they weren’t the first to take an interest in this stork — as, clearly, the bird had already been pierced by a spear. The 75cm (30in) projectile had been shot through one side of the stork’s neck and had exited cleanly through the other.
However, the bird itself wasn’t what caught German zoologists’ attention. Rather, it was that the spear in its neck was clearly crafted using Central African technology; the European stork had obviously been shot in Africa, prior to its return to Germany. And yet, against every odd, the stork still managed to fly thousands of miles back, with the weapon still lodged in its neck.
From then onward, this stork became known as the Pfeilstorch: the arrow stork. Here’s two things its discovery allowed us to learn about migratory birds.
1. The Physiology Of Migratory Flight
Although migration may seem an unremarkable fact of life today, almost all zoologists at the time hadn’t a clue where some species of birds were disappearing to in the winters. Working theories in the 1800s were absurd; some believed they hibernated in mud, or underwater, where humans couldn’t see them. Some people were even convinced that birds transformed into other animals.
As laughable as they were, these were the only explanations zoologists could think of. But the Pfeilstorch (or, rather, the giant spear in its neck) was the first tangible piece of evidence we had that could suggest otherwise. Only then did it become clear that some birds fly south in the winter — a fact that only a select few people considered prior to the Pfeilstorch’s discovery.
Even 200 years later, migration stands as one of the most fascinating adaptations in the animal kingdom, and the Pfeilstorch is one of the most impressive reminders of this fact. To understand its significance, it helps to understand how exactly birds evolved the ability to go on these impressive journeys.
It’s most important to understand that long-distance migration isn’t merely about a bird’s will to travel. It’s entirely impossible to achieve without the suite of profound physical adaptations it requires. Migratory birds, like the stork, had to evolve into what you could consider aerial athletes to efficiently fly these enormous distances.
Wing morphology plays an integral role in this process. As research from Functional Ecology notes, migratory birds, from storks to sparrows, tend to have long, narrow wings with high aspect ratios. This design allows them to generate considerably more lift relative to their drag than that of non-migratory birds.
Muscle endurance is just as essential a consideration in migration. The pectoral muscles that power birds’ wingbeats make up an enormous proportion of their total body mass. Research from The Journal of Experimental Biology attributes this to the fact that these muscles contain a high density of mitochondria and capillaries. This, in turn, enables the muscle to operate aerobically for long stretches without fatiguing.
But perhaps the most remarkable migratory adaptation is the way these birds store, and subsequently use, their energy. Right before their migratory journeys, many species go through a period of intense feeding, known as “hyperphagia.” As a classical study from The American Journal of Clinical Nutrition explains, this process allows birds to essentially double their body weight before migrating.
This fat is then stored in various crucial spots throughout the birds’ bodies: beneath their skin, packed in between all their muscles and even around their most vital internal organs. Given that fat is an energy-dense fuel, this gives birds double the amount of energy per gram of body weight than, say, what carbohydrates or proteins could. In other words, this means a small songbird that weighs in at as little as 20 grams could carry enough fat to fuel them throughout a nonstop transoceanic flight that takes several days.
In hindsight, what made the Pfeilstorch so extraordinary wasn’t just that it survived its return trip to Europe with a spear through its neck. Rather, it’s that its survival is one of the most perfect illustrations of the adaptations scientists would later identify as the keys to migration.
Its long, soaring wings are what allowed it to leverage rising columns of warm air with minimal effort. Its powerful breast muscles made hours upon hours of flight sustainable. The fat reserves it likely spent days building up before its departure gave it the energy to push on, despite being so gravely wounded. The very existence of the Pfeilstorch was proof that these birds were built, quite literally from the inside out, to endure one of the most demanding survival strategies in the natural world.
2. How Birds Know The Way
Regardless of how impressive their endurance capabilities are, migration doesn’t rest on a bird’s physiology alone. Of course, these birds also need to know where they’re going, too. And, once again, evolution offered birds a miraculous solution.
Magnetoreception, as explained by a study in Current Opinion in Neurobiology, refers to some migratory birds’ ability to physically sense the Earth’s magnetic field. The study describes how some birds possess light-sensitive proteins in their eyes, known as cryptochromes, that allow them to “see” magnetic fields as visual patterns or gradients.
This near unbelievable capability guides birds’ journeys in precisely the same way the direction of Earth’s magnetic fields guide a compass’s needle. In turn, birds can fly confidently in areas they’re unfamiliar with, as well as in conditions with poor visibility.
Other species instinctually rely on celestial cues in the same way early human explorers did. A 2017 study from Biological Rhythm Research notes that nocturnal migrants have been observed to navigate their journeys by the position of the stars. Some diurnal migrants, on the other hand, use the sun’s arc.
Even young birds, with no previous experience of migration, are innately capable of flying thousands upon thousands of miles, along the exact same routes that their ancestors did.
Research from the journal IBIS demonstrated that birds’ genetic programming seems to encode this ability. As a result, they’re born pre-programmed to know the direction, timing and distance of their migration journeys. And over countless generations, natural selection has fine-tuned these inherited maps; the routes that maximized species’ chances of survival were the ones that were maintained.
These many navigation systems serve as a bird’s repertoire of backup plans: when one cue fails, another steps in. They make it possible for a stork pierced by a massive, foreign spear to find its way back to Europe. That is, having multiple (perhaps even redundant) evolutionary tools were necessary for migration to become the reliable survival strategy it has been for centuries.
Modern science continues to uncover just how complex these adaptations are, and there are still facets of these impressive creatures’ journeys that we’re yet to learn. What is clear, however, is that migration is one of evolution’s most elegant solutions to a problem as old as life itself: how to survive in a changing world. And, ultimately, this is a truth that was first proven undeniable by the Pfeilstorch’s remarkable flight.
Are you an animal lover who owns a pet, perhaps even a bird? Take the science-backed Pet Personality Test to learn exactly how well you know your little friend.