Can You Find Your Way Home without your Sense of Sight? Neither Can Birds
Writer’s Comment: This article was the final assignment for UWP 102B (Writing in the Biological Sciences) and my first attempt at a journalistic piece on a scientific topic. The assignment was to report on the findings of a scientific study to a lay audience, in the style of a New York Times science section article. I decided to discuss the latest findings in avian magnetoreception, a topic I was completely unfamiliar with but found very fascinating. To learn more about the topic, I emailed researchers specializing in avian magnetoreception in Germany and was thrilled to get responses that I could incorporate into the piece. I thoroughly enjoyed writing this article and hope you enjoy reading it. I would like to sincerely thank Professor Jared Haynes for all his guidance and encouragement to pursue a career in science journalism.
Instructor’s Comment: Nancy is a writer with a strong instinct for her audience. During my course, Nancy demonstrated her ability to shift styles between the audience for her literature review (given an Honorable Mention in this volume) and the audience for this article. In the literature review, she conveyed technical information on bird navigation to a sophisticated audience. In the article published below, she adapted material from the literature review and completely re-visioned it for the kind of reader that the New York Times attracts to its science section. She grabs the reader with an interesting title and an engaging opening sentence, she uses the short paragraphs typical of such newspaper articles, and she feeds the reader interesting material without much technical terminology. I urge students who choose this assignment to write or call up researchers to get quotations about the research, since this is also typical of New York Times science articles; Nancy was one of the few who did so, and the quotations add interest and liveliness to her article.
—Jared Haynes, University Writing Program
Fitting European robins with frosted and clear goggles may seem silly, but researcher Katrin Stapput and her colleagues have recently made critical advances in the field of bird navigation using this method.
In this study, conducted in July of this year at Goethe-Universitat in Frankfurt, Germany, the goggles were used to measure how quality of vision affects a bird’s ability to navigate using Earth’s magnetic field.
Birds use a receptor to gather information from the magnetic field, a technique known as avian magnetoreception. For years, researchers have been well aware of its presence and function in the avian system. Nonetheless, numerous questions remain, as the mechanics of the system are still extremely uncertain.
It is thought that the presence of light stimulates reactions in the eye that let birds actually see the magnetic field. Specialized molecules in the eye detect the magnetic field as patterns of light and dark, which are superimposed over the images they see with their regular vision. Birds can navigate using the magnetic field as a compass for traversing vast distances during migration, or even for short distance movements around their home territory.
Researchers can test this form of navigation by manipulating variables and monitoring how these changes alter the orientation the birds choose in comparison to the orientation that might be expected when the birds’ magnetic compass is unaffected.
Stapput, who was a graduate student in Dr. Roswitha Wiltschko’s lab during this study, tested 12 European robins, a bird species known to migrate to avoid harsh winters. Every bird was tested in each of the four study conditions.
In one of the settings, birds had only the left eye covered with a frosted goggle that distorted vision. In another, the right eye alone had a frosted goggle. In a separate test, birds without goggles were subjected to an electromagnetic field, which interrupts the reactions in the eye. This test confirmed that the birds were using information gained from the eyes to orient, rather than from a different sensory mechanism. Finally, as a control, the birds were allowed to navigate without any interference from goggles or electromagnetic fields.
“As to problems doing the research, there were not any unusual ones. Of course the birds did not enjoy wearing goggles,” said Dr. Roswitha Wiltchko. “But they got used to it. And besides, they did not have to wear the goggles very long—about 1 1/2 h every second or third day. At the end of the experiments, we could release them all with two intact, shiny eyes back into the wild.”
So although the goggles did not cause harm, while worn they did affect the birds’ vision. The frosted goggles let light into the birds’ eyes, but they also blurred the birds’ vision greatly. The electromagnetic field interrupted reactions in the eyes and made it impossible for birds to use their vision to navigate. Applying the electromagnetic field allowed researchers to confirm that the birds were relying on their eyes, rather than some other method, to navigate.
This study, published in Current Biology, showed that birds indicated a preference by consistently orienting toward their northerly migratory direction under control conditions and also when only the left eye had a frosted covering. When subjected to electromagnetic fields and when only the right eye was covered with frosted goggles, birds showed confusion, lacked a preference for their normal northern migratory direction, and oriented in all different directions.
The difference in orientation when separate eyes were frosted supports a previous finding that birds display lateralization, meaning that the left and the right eye perform different functions. Both studies suggest that a bird’s use of the magnetic field is lateralized to the right eye, meaning that the right eye functions comparatively more than the left eye in magnetic field-based navigation.
A similar previous study in which birds wore eye patches that completely blocked out light and vision resulted in the birds being unable to navigate. This study had led to a crucial question: is it light alone that is needed for magnetoreception, or do the birds actually need to see images?
Stapput’s study is the first to show that for avian magnetoreception to work properly, a quality image that is not blurred or distorted is required in addition to light.
Birds need to have fully functional vision that lets them see objects as well as light patterns. The necessity of unobstructed vision, rather that light alone, to properly navigate implies that birds must somehow use the contours of objects to orient to the magnetic field.
Thorsten Ritz and colleagues, researchers in magnetoreception at the University of Illinois, suggest that birds need the outlines of objects to distinguish two kinds of information––that which comes from light hitting objects and that which birds obtain from the magnetic field––both of which go into the eye.
With normal vision, birds can easily separate this information. Objects in the visual field have sharply contrasting light patterns, whereas changes in the magnetic field are smooth and gradual.
However, with the frosted lenses on the right eye, the birds could not use the difference of contrasting and smooth patterning to understand what information was coming from the magnetic field and what was coming from objects in the field. This confusion could explain why they could not navigate when the right eye vision was blurred.
Another explanation is that birds which can’t see object edges due to blurred vision in their right eye will switch from dominantly using their right eye to using their left eye in navigation. This causes problems because the left eye doesn’t usually function in interpreting magnetic field information and therefore is limited in its ability to discern the magnetic field.
Despite these limitations, eyes provide the birds with a very effective tool that is used not only in migrating over vast plains or oceans, but also in simpler tasks like finding their way back to the nest.
In fact, navigation is so important in birds that they have yet another method for detecting the magnetic field––their beaks.
Two primary hypotheses have arisen to explain how birds can receive magnetic field information. These are visual detection through molecules in the eye (called radical pair mechanism) and an iron mineral mechanism in the beak. Both have been supported through numerous past studies.
The differences in functionality of the two systems are discussed by Dr. Wiltschko.
“The two systems measure different qualities of the geomagnetic field—the radical pair processes in the eye provide birds with information on magnetic directions, and the iron-based receptors in the beak provide them with information on magnetic intensity—these are two different variables of the geomagnetic field.”
Dr. Wiltschko equated this system to a parallel in the senses of hearing and touch. She explained that although both hearing and touch operate as senses, they supply different information. Similarly, the eye and beak both act as magnetic sensors, but the specific information each provides is unique and therefore measured with different methods.
Yet, not all researchers readily accept that both the beak and the eye serve as magnetoreception systems, as Ken Lohmann, a professor at the University of North Carolina, explains. Currently, Katrin Stapput works as a post doctorate associate in Lohmann’s lab.
“Many researchers see no conflict between the two ideas and think that two different magnetoreception systems exist in birds,” states Lohmann. “However, a few disagree and maintain that all magnetoreception is based on magnetite, with no exceptions.”
Katrin Stapput’s research with goggles does lend support for the radical pair mechanism in birds’ eyes, but at the same time it does not rule out the iron mineral hypothesis as a contributing mechanism. Possibly, these two mechanisms operate completely independently of one another and therefore are not conflicting theories at all.
Dominick Heyers, a neurobiologist who recently conducted a study of the trigeminal nerve that transmits magnetic information to the beak, explains. “My study on the trigeminal system and their [Katrin Stapput and colleagues] study on detection of directional information do not contradict. They were looking at the vision-based compass system and added some important findings supporting the theory of a vision-based compass system.”
Stapput’s finding regarding the necessity of images in navigation redefines researchers’ ideas about how compass navigation works in birds. So, as strange as they may look, simple tools like bird goggles can provide us with valuable information.
Who knows what new contraptions researchers will be designing in future studies to help us understand the complex and magnetic world a bird sees?
Heyers D, Zapka M, Hoffmeister M, Wild JM, and Mouritsen H. 2010.
Magnetic field changes activate the trigeminal brainstem complex in a migratory bird. PNAS 107 (20): 9394-9399.
Ritz T, Adem S, Schulten K. 2000. A model for photoreceptor-based magnetoreception in birds. Biophysical Journal 78: 707-718.
Stapput et al., Magnetoreception of Directional Information in Birds Requires Nondegraded Vision, Current Biology (2010), doi:10.1016/j.cub.2010.05.070