Earth’s Magnetic Field: A Factor in the Migration of Birds
Geography 101 – Physical Geography
April 23, 2007
“Migrating birds rely on various senses and behaviors to guide them” in their navigation process on the Earth (Sibley, 2001, p. 63). These senses include their sight, as in noting where the sun and moon are during the day, where the stars are at night, and where particular familiar landmarks are on the Earth’s surface as they fly. They also use their hearing as well as sensing the Earth’s magnetic field. They need to learn at a young age how to navigate with these tools or for some species these tools may be with them when they are born. The tool I am going to focus mainly on in this paper is the reliability of the magnetic field of the Earth in helping birds orient and navigate during their travels. They may have tiny magnets, or groups of magnets, inside them that they use to find direction, or they may “see” the magnetic field around them.
Is our Earth Alive?
Is our Earth living or is it abiotic? In the original suggestion for the title of this paper the second line was: “An Abiotic Factor in the Migration of Birds.” I looked up “Abiotic.” Its definition: Abiotic - Nonliving: The abiotic factors of the environment include light, temperature, and atmospheric gases (American Heritage Dictionaries, 2007). I’m going to be somewhat un-scientific in that I am going to think, as I write this paper, that Earth is alive. Why not? I always do and why should I stop thinking that way when writing this paper? To a butterfly sitting on a tree, the tree does not appear alive. I will also consider, since the Earth’s magnetic field is part of the Earth that it is a LIVING part of Earth. In an article by James Lovelock in 2003 he talks about his Gaia Theory, that our Earth is a living Earth. He says that Newton, Hutton, Huxley and Vernadsky all thought of the Earth as a living being, and the writer William Golding called it “Gaia.” Toward the end of the article Lovelock points out that in 2001, “more than a thousand delegates [at a scientific conference in Amsterdam] signed a declaration that started with the statement: ‘The Earth System behaves as a single, self-regulating system comprised of physical, chemical, biological and human components’” (Lovelock, 2003). I will capitalize “the Earth” and “Earth” as I believe it is fine either way and I’ve seen it capitalized in many of the articles I have read recently and I prefer the capitalization of the word.
Our Earth and its Magnetic Field
Our Earth contains and is surrounded by a magnetic field, just as a magnet is surrounded by a magnetic field. Our Earth IS a huge magnet. This magnetic field is invisible and it is all around us all of the time. The Earth’s magnet field, sometimes called the Earth’s magnetosphere or the geomagnetic field is huge compared to our Earth (Chaisson, Mcmillan 2004, p. 151). See Figure 5.20 from, Astronomy, A Beginner’s Guide to the Universe, to see a representation of our Earth’s magnetic field on page 11 of my paper. According to Kathryn Brown in an article in 2001, the Earth contains a constantly churning core and mantle which is what “casts a magnetic field across the planet’s surface” (Brown, 2001).
There are more than 650 bird species in North America. About 75 percent of them migrate in one form or another (Sibley, 2001, p. 61). There has been much research into why birds migrate and how they manage to orient themselves over their long flights. Some birds fly over oceans, deserts and mountains without stopping. Other birds make short hops and pause at times to eat food and “refuel” sometimes for up to several days, and then they continue with their flight (Sibley, 2001, p. 60). In the northern hemisphere of Earth these migrating birds fly north in the spring to their summer breeding grounds. They stay in the same place every year and they rear their young where there is an abundance of food, especially insects for those birds who eat insects. In the autumn they head back south to their winter home because as the temperature drops the food and insect supply also drops in their summer home (Sibley, 2001, p. 60).
Different bird species fly at different times of the day and in different ways. There are terms for these birds (Sibley, 2001, p. 60-61). Diurnal migrants fly on the thermal winds and updrafts. I have included an image from the book The Sibley Guide to Bird Life & Behavior on page 12 of my paper that demonstrates “riding thermals.” The crepuscular birds start their daily travels just before dawn and they fly mostly in the early morning. Nocturnal migrants fly from just after sunset and into the night. Long-distance migrants are birds that fly great distances, over oceans and deserts and from one continent to another. Short-distance migrants are those that usually stay within one continent. There are even some species where only some of the population migrates back to their warmer homes in the winter and some remain in their winter breeding grounds all winter (Sibley, 2001, p. 60-61).
Migratory Birds Using the Earth’s Magnetic Field
“Birds, fish, crustaceans, and a host of other animals appear to use regional variations in the magnetic field, along with sensory cues such as sight and sound, to navigate” (Brown, 2001). “Migratory birds can use various orientation cues during their migratory journeys.” These include, “celestial and geomagnetic information for orientation” (Akesson, Alerstam, Muheim, 2003).
Akesson, Alerstam and Muheim talk about their development of a computer program that simulated “migration routes starting from any location on Earth into a chosen direction.” Of course, they knew generally which routes certain species of birds followed in order to correctly make their calculations. There is a map on page 347 of the article which shows the “hypothetical magnetic compass course a Savannah sparrow has to follow in order to reach the species-specific wintering area” (Akesson, et al., 2003). See this map on page 11 of my paper.
There is a “distinction … between orientation and navigation (Sibley, 2001, p. 63).” “Orientation is the ability of a bird to use an internal compass to accurately align itself in an appropriate direction when released into unfamiliar surroundings. Navigation is the ability to find a specific geographical location, from a known starting point. Navigation is usually thought to require some form of internal map, plus the ability to orient (Sibley, 2001, p. 64).”
According to Sibley birds can orient themselves when released during the daylight, in the night sky or with other orientation cues (2001, p. 64-65). Sibley says in his book that many experiments have been performed to see if birds really do use the Earth’s magnetic field to orient themselves. He says, “…it is now viewed by many as the basic avian compass, particularly for long-distance migrants” (2001, p. 65). He also points out that birds notice anomalies in the earth’s magnetic field and use these during navigation. Many “researchers believe that Earth’s magnetic field may provide a basis for avian navigation, as well as its role in orientation” (Sibley, 2001, p. 65).
I am including on page 13-14 of my paper a map of the U.S. and Canada showing the four major flyways of waterfowl in North America. The four flyways concept is an older concept especially since now there has been movement of birds recorded between flyways during migration and dispersal. The four flyways concept is still “useful and is the basis for much waterfowl management” (Sibley, 2001, 204). I have also included a map from the same book of the migration path of Swainson’s Hawk. (Sibley, 2001, p. 223). This second migration map is on page 13-14 of this paper. I’ve also included images of the two birds mentioned in the text that accompanies the maps.
There have been many theories that go back for over a century about how birds find their way when they navigate. According to an article by David Meshoulan (2000), in February of 2000 a group of people in Illinois led by biophysicist Klaus Schulten, proposed that some “birds can ‘see’ the earth’s magnetic field in dim blue or green light.” They then use what they see to find their way. Another researcher mentioned in Meshoulan’s article, Wolfgang Wiltschko, studied some birds called the silvereye from Australia. He found that they were confused in red light but “could orient themselves correctly under blue or green light” (Meshoulan, 2000).
In this same article a theory is also mentioned where some scientists believe that there are tiny magnetite crystals in the brains of birds that can record “even small changes in the strength of a magnetic field” (Meshoulan, 2000).
In an article by Jane Qui (2005) she talks about some scientists from about a hundred years ago who thought perhaps birds had an “inner magnetic sense, like a compass needle.” In 2005 it was found that there is indeed “magnetite, or iron oxide, in the brains of some bird species.” And… “The magnetite is thought to act as a tiny compass” (Qui, 2005).
Magnetite deposits “have been found in a variety of higher animals that exhibit magnetic sensitivity, including fish, newts, birds, and mammals.” “‘Superparamagnetic magnetite’ has been located in the beaks of homing pigeons.” Superparamagnetic magnetite is quite tiny and is thought to be so sensitive that any animal or bird with it can detect even minute changes in the geomagnetic field (Freake, Muheim, & Phillips, 2006).
In an article in Science on April 16, 2004, by Cochran, Mouritsen & Wikelski, it is noted that these scientists did studies on night migratory songbirds. They used control birds as well as experimental birds. When released into the dusk sky, the control birds all flew in the directions that were expected of them. Some of the experimental birds were placed in a cage with artificial coil system magnetic fields around them that were dissimilar to the natural magnetic fields of Earth. Some of the birds were confused and flew in the wrong direction when they were freed in the evening. Some of the birds flew to a nearby woods, waited one to six days until they became correctly oriented, then flew off in the correct direction. Some of the birds that flew in the wrong direction were “stuck” in the incorrect direction as set up by the artificial coil system for some time after they started their flight. They totally ignored the cues of the stars that they could plainly see and continued to remain “stuck” for about a day (Cochran, Mouritsen, Wikelski, 2004). These birds released from the cages did eventually fly in the proper direction that they were “expected to” fly. Even though they started out in the wrong direction they recalibrated their magnetic compasses each day so that they ultimately found their correct orientation and migratory direction.
I have included an image from the article, called Figure 2, which shows a map of the area where the experiments had taken place; it is on page 15 of my paper. The black lines show the flights of the control birds in their natural flight patterns. It also shows the red arrows of the birds that flew in the wrong direction. The yellow lines show where the experimental birds corrected their flights by recognizing the correct magnetic heading after some time. The white arrows on the left part of the image show the birds that waited several days in the woods before taking off. They did a good job! I notice one poor bird on the right side of the image that almost went totally in the opposite direction but then corrected his flight on the second day. It looks like the scientists watched some birds longer than others, and lost track of some of them as indicated by the broken lines at the ends of some of the flights. The final sentence in the article says, “We suggest that the simple yet reliable twilight-calibrated magnetic compass may be used by many … species of night migratory birds in the wild.” It seems the birds recalibrate their compasses every twenty-four hours (Cochran, Mouritsen, Wikelski, 2004).
“…the magnetic map hypothesis … proposes that an animal learns the alignment, and possibly the steepness, of one or more of the magnetic gradients [of the Earth] as it moves within, and occasionally beyond, the home range” (Freake, Muheim, & Phillips, 2006). It is pointed out in the article, Magnetic Maps in Animals (2006), that birds can take note of the fact that the strength of the magnetic energy of the Earth changes at different points on the Earth. The authors of this article call the differences “global gradients” and state that the strength of the magnetic field of the Earth is weaker at the “magnetic equator” and stronger toward the poles. I am including a map of the Earth from this article on page 16 of my paper to illustrate this point. This map shows that the intensity of the geomagnetic field is stronger at the poles. Another map from the same article also interested me a great deal. It is also on page 16 of my paper and actually shows the variation in strength of the magnetic field in units of measurement called nT (nanoTesla). The entire geomagnetic field is about 50,000 nT (Freake et al., 2006). The birds actually need to learn the variations in the magnetic field to find their way in the spring and again in the autumn. They either have to learn it or they “know” it inherently (Freake et al., 2006).
The Sun’s solar wind can cause changes in the variations in the gradients of the Earth’s magnetosphere. This can even cause daily fluctuations which can “introduce significant errors in fine-scale map estimates.” In this case birds flying over short distances may “avoid taking map measurements during such times” (Freake et al., 2006).
The Tasmanian race of the Australian silvereye birds has their summer breeding grounds in the southern island of Tasmania in Australia. They migrate up the western coast of New South Wales and even up into Queensland in the southern winters. “Direct evidence that adult birds rely on magnetic map information comes from a study that tested the birds’ responses to the experimental manipulations of the magnetic field that simulated displacements to different locations along the migratory path” (Freake et al., 2006). Some of the birds were moved from their breeding grounds in Tasmania to Armidale, New South Wales. They were moved North-North east. Some of them were placed in “artificially altered magnetic fields simulating displacements to areas north (SimN) or south (SimS) of Armidale” (Freake et al., 2006). The intensity and inclination of the magnet fields were different for different birds. The birds were exposed to the “altered magnetic field values” for at least six days. Birds in the SimS “condition” flew the same way the control birds did. Most of the birds in the SimN “condition” were confused. It was found later on that the adult birds were confused and young birds that had not migrated before were not effected (Freake et al., 2006). I have included the map illustrating this experiment on page 17 of my paper.
There seems to be a lot of experimenting going on with birds. One I didn’t mention before and won’t talk about in great detail is an experiment done with some pied flycatchers that fly from Germany to places in Africa during the Northern winters. The scientists again caught some birds, this time in Sweden and placed some of them in artificial magnetic fields; some of them were the control group and were held at what they call the “Sweden” value. They studied whether the birds in the artificial “Egypt” magnetic field would think they were actually in Egypt and eat lots of food just as they would when in the real Egypt before their long flight over the Sahara Desert. The control group did not “increase in body mass” but the experimental group did. The birds thought they were in Egypt even though they were actually in Sweden (Freake et al., 2006).
Birds are flying all over the world every day. I never really thought much about them before doing my research and writing this paper. They were just some creature out there that I would enjoy listening to especially upon waking. I knew that they migrated and all that but I never really gave it much thought. When I read through the list of suggested topics for a research paper it was the thought of the Earth’s magnetic field that attracted me. I now believe that birds do use the magnetosphere to orient themselves during their flights and during their navigation to far-off and not-so-far-off places. I now know that there are lots of people out there trying to figure out why birds do what they do. It’s very interesting that birds can not easily be studied in a laboratory setting; they need to be studied in their natural habitat as much as possible. I picture scientists out there with their binoculars following the movements of the birds that they are studying. I know they capture the birds and band the birds but the picture of the scientists with binoculars won’t leave my head.
Åkesson, S., Alerstam, T., & Muheim, R. (2003). Compass Orientation and Possible Migration routes of Passerine Birds at High Arctic Latitudes. OIKOS: A Journal of Ecology, 103(2), 341-349.
American Heritage Dictionaries. (2007). Abiotic: Definition and Much More from Answers.com. Retrieved March 23, 2007, from Answers Corporation Web site: http://www.answers.com/topic/abiotic
Brown, K. (2001). Animal Magnetism Guides Migration. Science, 294(5541), 283-284.
Chaisson, E., & McMillan, S. (2004). Astronomy: A beginner’s guide to the Universe. Upper Saddle River,NJ: Pearson/Prentice Hall.
Cochran, W. W., Mouritsen, H., & Wikelski, M. (2004). Migrating Songbirds Recalibrate Their Magnetic compass Daily from Twilight Cues. Science, 304(5669), 405-408.
Freake, M.J., Muheim, R., & Phillips, J.B. (2006). Magnetic maps in animals: a theory comes of age? The Quarterly Review of Biology, 81(4), 327-347.
Lovelock, J. (2003). The Living Earth. Nature, 426(6968), 769-770.
Meshoulam, D. (2000). Magnetic Eyes: How Birds May Find Their Way with Quantum Physics. The Sciences, 40(3), 9-10.
Qiu, J. (2005). Flight of the Navigators. Nature, 437(7060), 804-806.
Sibley, D. A. (2001). The Sibley Guide to Bird Life & Behavior. New York: Alfred A. Knopf.
Green-Winged Teal. (2007, April 12). In Wikipedia, The Free Encyclopedia. Retrieved April 9, 2007, from http://en.wikipedia.org/wiki/Green-winged_teal
Swainson’s Hawk. (2007, April 14). In Wikipedia, The Free Encyclopedia. Retrieved April 9, 2007, from http://en.wikipedia.org/wiki/Swainson%E2%80%99s_hawk
I wrote this paper. Please do not use it and call it your own. You may use it as a reference. You should cite it properly if you do use it. You can also find the articles above and use them for refernece purposes. As I do with all my college papers, I worked my butt off on this one. I got an A!
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