Written by Jason Pyron
As we head into the fall, bird lovers everywhere are anticipating the upcoming migration and the different varieties of birds that they may be able to see in their backyards or at their feeders. This leads us to reflect on the questions that have been asked throughout human history: Where do birds go in the winter? How do they get there? Why do they leave? What do we know about this process and what do we still not fully understand? This is the first in a series of articles that addresses these questions.
Before we dive into these topics, we need to explore how human understanding of migration has evolved over time. In ancient times, the prevailing theories that sought to explain the disappearance of birds in the winter would be considered bizarre by today’s standards. Aristotle popularized several different beliefs about birds during his time that would shape human thought around migration for hundreds of years. One of these theories was that bird bodies transformed throughout the year. This was not as simple as birds molting into different plumages, but instead was that birds changed to different species entirely. Examples of this were the Common Redstart, present in the spring and summer would gradually gray and shift its orange plumage to its head and become smaller, or the Garden Warbler would change from brown to gray and develop a brown cap in the winter. We know today that he was confusing the Redstart and the European Robin, and the Garden Warbler with the female Eurasian Blackcap, both of which are winter residents in Greece.
A second theory Aristotle proposed was that birds, specifically small birds like barn swallows, hibernate. Since other animals were known to hibernate during the winter, he applied the same logic that birds would find crevices, holes in trees, or other places to endure the cold, food scarce time of year. There would be no way that Aristotle would have been able to confirm that these birds were showing up in Africa weeks after their trans-Mediterranean flight. Human travel back then was extremely slow, and there was very little if any interaction between the Greeks and people of central and southern Africa at this time. This theory was commonly accepted, and despite challenges by some people throughout the centuries, it had a resurgence in Europe in the 1300’s when it eventually evolved to a belief that birds shed all their feathers and lived in holes in the ground during the winter.
In 1555, a Swedish Bishop living in Rome, Olaus Magnus, published a book titled “Description of the Northern Peoples” that documented the history, natural history, and folklore of Sweden. One of the claims in the book was that swallows would gather in large flocks in autumn and plunge into the mud and water residing there during the winter and emerge in spring. He claimed that inexperienced fishermen pulled swallows up in their nets, resulting in the death of the birds. Since swallows were often seen skimming closely to the top of bodies of water, the next logical step was that birds could survive under the water. This book, which also popularized the idea of the existence of sea monsters such as the Kraken, was translated into several different languages, and led to the spread of this theory.
By the end of the 1600’s, Charles Morton, a scientist and educator published a paper that stated changes in temperatures and food were motivation for birds to migrate to other areas. He rejected the possibility of birds hibernating underwater since they cannot breathe water. However, the migration that he proposed was that birds migrated to the moon! To his credit, this seemed logical since there was no evidence that proved birds were living underwater, and some unfortunate experiments with swallows in ice houses proved they could not tolerate the cold. It was becoming clearer that birds were flying somewhere, and they were observed flying in front of the moon in large flocks at night. At the time, people believed the moon and other planets were habitable and did not understand the distances and conditions that birds would have to endure on such a trip. We now know that the bird with the highest migration altitude is the Bar Headed Goose which must cross the Himalaya Mountains. This bird maintains a height of 23,000 feet which is still about 238,896 miles away from the moon. Additionally, birds would have to fly at over 25,000 mph to escape the gravitational field of earth, so it is safe to say we are still a long way off from birds in space. While these theories might seem ridiculous to modern audiences, if we were to try to explain our current understanding of bird migration to the ancients, they would likely find it easier to believe that birds flew to the moon.
Beginning in the 1800’s our understanding of migration began to change significantly. Evidence began to increase that made it clear birds could fly great distances. An example of this was a White Stork that was shot by a hunter in northern Germany. It was discovered to have a spear lodged in its body. The spear was determined to be made from wood native to central Africa and matched the types of spears used by the people there. This served as evidence that this “Pfeilstorch” (Arrow Stork) was capable of not only transcontinental flight, but a transcontinental flight with a spear embedded in its neck! Since 1822, 25 separate cases of migratory storks with African spears embedded in their bodies have been observed.
By the early 1900’s the use of metal bands attached to bird legs for purposes of tracking them began in Europe and the United States, and our understanding of bird migration began to quickly evolve. However, even in modern times some migration myths still persist. One of these myths is that hummingbirds actually migrate on the backs of geese in order to arrive in Central and South America. Although no one is sure when or where this common folklore originated, it is not true. Even if these birds migrated at the same time, the logistics involved just don’t add up. Of the many humans that have monitored geese using aircraft, none have spotted little green stowaways bound for warmer climates.
Our next article we will explore the modern understanding of the science of bird migration.
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Richard Armstrong, Ancient Explanations of Bird Migrations https://engines.egr.uh.edu/episode/2228
Lucy Cooke - 3 Bizarre (and Delightful) Ancient Theories About Bird Migration, https://www.ted.com/talks/lucy_cooke_3_bizarre_and_delightful_ancient_theories_about_bird_migration?language=en Rebecca Heisman, A Brief History of How Scientists Have Learned About Bird Migration