Written by: Bethany Beekly
Editors: Christina Del Greco, Henry Ertl, Jennifer Baker, and Madeline Barron
I recently traveled to Seattle for the wedding of a high school friend. As I prepared for my first flight since the pandemic began—dreading the long lines, the awkward shoeless dance through security, and the inevitable 4.5 hour battle with the child kicking me in the back—my thoughts drifted to the remarkable flights made by hundreds of species of migratory birds and the relative ease with which they make them.
The Arctic tern travels back and forth between the North and South Poles each year. It must fly approximately 45,000 km (28,000 mi) each way and is virtually never seen touching down except during breeding season. Many hummingbirds travel between Central and South America to locations in the United States for the breeding season, flying as much as 37 km (23 mi) per day despite weighing about as much as a penny. And even flightless birds make impressive journeys: climate change has altered the Adelie penguin’s range such that it now walks almost 13,000 km (8,078 mi) between eastern Antarctic breeding sites and the Ross Sea, where it spends the winter.
Physical feats of endurance notwithstanding, one of the most extraordinary aspects of long-distance avian migration is the navigational ability of birds. Many birds will return to the same spot—sometimes literally the same tree or burrow—to breed year after year. Although they still struggle to understand precisely how it works, scientists have learned that birds’ uncanny sense of direction depends largely on magnetoreception, or the ability to detect the Earth’s magnetic field. There are two different types of magnetoreception: fixed orientation and compass orientation. The fixed orientation response, which seems to sense magnetic intensity, likely allows the bird to understand its latitudinal position, facilitating longer-distance navigation. Compass orientation, on the other hand, tells the bird its longitudinal position so it can discern whether it’s moving “poleward” or “equatorward.” (Typically, birds move away from the equator during the spring migration and towards it in the fall.)
While mechanisms of the fixed orientation response remain unclear, recent research has helped elucidate the compass orientation response, particularly how magnetoreception in the retina (the most interior layer of the eye) may serve as a possible mechanism. Retinal photoreceptor cells detect light and transmit visual information to the brain; they contain a special class of proteins called cryptochromes that are believed to be capable of responding to magnetic fields. Humans and other animals have these proteins, too—they are important for the regulation of circadian rhythms. However, a unique subtype of cryptochrome, called Cry4a, has been found in birds. Cry4a lacks a clear circadian function but still has the necessary biochemistry for magnetoreception.
For magnetoreception, the most significant biochemical feature of cryptochrome proteins, including Cry4a, is their unique potential to host radical pairs. A radical is an atom, molecule, or ion with an odd number of electrons. Sometimes, two such unpaired electrons can influence each other’s behavior, forming what is called a radical pair. Radical pairs are unstable and are generally found in short-lived intermediate states that form during the process of a chemical reaction, similar to the self-assembly process for furniture. Imagine that as you are building a dresser, you screw in the screws just partway at first, to make sure everything lines up. It’s close to being a dresser, but it can’t stand on its own or hold anything without collapsing. It could just as easily fall apart as go on to become a proper piece of furniture. Since electrons carry a negative charge, the movement of radical pairs results in a magnetic field, albeit a tiny one, that can interact with a second magnetic field—for instance, that of the Earth itself. These interactions can influence whether the unstable radical-pair intermediate reverts back to the starting materials or completes the reaction to form the final product—does it fall apart into a bunch of boards, or does it become a functional dresser?
When high-energy light, like the blue and ultraviolet wavelengths emitted by the sun, hits Cry4a in the retina, it sets off a chemical reaction that results in a radical-pair intermediate. The angle of the magnetic field, which is based on the relative distance from the equator, interacts with the radical pair, affecting whether the chemical reaction progresses to final products or reverts to the original starting materials. It is believed that the ratio of “starting materials” to “final products” somehow generates a signal that photoreceptor cells in birds’ eyes use to communicate with the brain. The ratio of starting materials to final chemical product is interpreted by the brain to ascertain the bird’s location.
While I find these topics fascinating, I personally study neuroscience, circadian rhythms, and reproduction. I’ve become particularly interested in magnetoreception in the retina because I wonder if it not only helps birds navigate during their journey, but also tells them when they’re in the right place to reproduce. In animals that only reproduce at a certain time of year, fertility requires light hitting the retina in extremely precise ways. But what if seasonal information isn’t enough? What if the reproductive system also needs a signal telling the brain that, with respect to the Earth’s magnetic field, the bird has arrived at the best location for rearing its offspring? If this were true, it could teach us why some birds don’t breed well in captivity and help us save endangered species. As far as I can tell, this is a relatively unexplored topic, so this should all be taken as exactly what it is: the speculation of a doctoral student. But I believe it’s an avenue worth exploring, because a world without the biodiversity of birds that exists today would be a far less beautiful world in which to navigate the everyday inconveniences of things like airport security.
Cramp, S., The Birds of the Western Palearctic. Handbook of the Birds of Europe the Middle East and North Africa. Vol. I. 1985: Oxford University Press, 1985
Kaufman, K., Lives of North American Birds. 2005, Houghton Mifflin Co.: Boston, MA.
Wiltschko, R. and W. Wiltschko, Magnetoreception in birds. Journal of The Royal Society Interface, 2019. 16(158).
Bethany Beekly is a Neuroscience student in the Elias Lab (UM Department of Molecular and Integrative Physiology). She studies the interplay between sleep and the neuroendocrinology of reproduction. She is also heavily involved with the Graduate Employee Organization (GEO), serving as Steward for the Neuroscience Program and the Chair of the Climate Caucus. When she isn’t doing science or fighting for worker’s rights, she can often be found outside hiking, camping, reading, doing yoga, or just sitting with a cup of coffee listening to the birds.