Photo: Ring-billed Gull, Sandra Wilbur/GBBC
Deidra Bryant
Have you ever wondered how birds are able to make their way to their fall/winter migration location? Migration has to be one of the most fascinating topics in ornithology. Primarily because of the avian physiology that works overtime to get them to their ultimate destination and how it may possibly be connected to something as ancient as the earth’s magnetic field.
Some birds, like the Sandhill Crane (Antigone canadensis) and Snow Goose (Anser caerulescens), learn their migration routes from the adults as they travel in family groups, while others may use their sight to note various geographic landscapes and bodies of water to let them know that they are on the right track. However, what I found most interesting were studies that investigated species that may use special chemicals in their eyes, magnetite in their beaks, or small currents generated in the bird’s inner ear as it navigates the planet’s magnetic field. These internal compasses allow them to orient themselves in locations that do not have obvious landmarks (such as oceans) even at night. However, after more than half a century of research on this topic, scientists are STILL unable to understand this “sixth sense”.
In the last couple of years, scientists in Europe have published articles explaining how a protein called Cryptochrome 4 or “Cry4” found in the retinas of Zebra Finches (Taeniopygia guttata) and European Robins (Erithacus rubecula) may allow them to “see” magnetic fields like a heads up display on a plane. They believed that changes in the concentration of this protein may affect light-sensitive cells within the eye, causing the bird’s view of the sky to become lighter or darker depending on the location and power of the magnetic field. The problem here is that scientists are now doubtful of the methods used to test this protein in the past. For instance, because they are light-sensitive, it does not explain their ability to navigate the dark NIGHT skies across the open seas. Another issue is that most of the studies that tested this chemical were only tested in a lab, and when scientists like Henrik Mouritsen and his team at the University of Oldenburg tried to replicate the original methods used to test magnetoreception, their results differed from the historical literature. Lastly, the magnetic fields used in the original study were much stronger than those generated by the planet. So, it begs the question: would the same finding apply to Earth-strength fields in a migrating bird?
Ring-billed Gulls, Sandra Wilbur/GBBC
As for the hypothesis that magnetite, a ferromagnetic mineral, may be present in the beaks of birds such as the messenger pigeon (Columba livia domestica), and even Ring-billed Gulls (per the 1972 study by William E. Southern), results have only been speculative. Current data shows that these birds may have a magnetic receptor in their body, but we don’t quite know how it works. For example, unlike the compasses people use, scientists believe certain birds are able to detect the axis of the magnetic field but aren’t able to distinguish between the north and south poles. Since magnetite crystals would respond to a flipped field like a compass needle, a bird should easily notice the difference and adjust accordingly. But in previous studies, birds only knew which direction the closest pole was but couldn't distinguish between north and south. So, when scientists inverted the magnetic field in the lab, they didn’t notice the change and continued to fly in the same direction. Another major issue with this idea of “trace magnetite within the beaks” is that no one has ever seen these iron crystals, even under a microscope, and even if they eventually did, much work would have to be done to prove that it has any relevance to the species’ nervous system and ultimately having it translate to directions.
Australian neuroscientist David Keays is currently testing a theory first proposed in the late 19th century that hypothesizes that tiny electric currents are generated in the bird’s ear through electromagnetic induction as they move through the planet’s magnetic field. Using an apparatus he built by running electricity through Helmholtz coils, he created a magnetic field to test the magnetic sense of pigeons. Essentially, he analyzes how neurons in the pigeons’ brains respond since extremely sensitive receptors in their inner ear would pick up the small induced voltages and send signals to the brain. He also believes that the rapid head turning in messenger pigeons during flight may serve to boost the voltage in the birds’ ears.
It is still a mystery as to how certain species are able to know just where to go every year, even when they are very young. This innate ability is still being explored.