What do we know about the Monarch butterfly's migration?
We know that millions of Monarchs that emerge as naive adults in August and September migrate from central and eastern United States and Canada to overwintering roosts in Mexico. The distances covered by the migrants are often more than 2,000 miles, and, remarkably, the roost sites they seek are located on just a few mountain tops in central Mexico. If we look at a map, we can see that these butterflies end up wintering in places that are south and west of where they started their journey. How do they get there? Clearly, not all of the Monarchs can reach the roost sites by simply flying straight south or southwest.
What are the questions?
The great Monarch mystery is: How do inexperienced Monarchs from Colorado to New England and the Canadian provinces all find the same traditional roosts in Mexico each year? What environmental information is used by Monarchs to guide their migratory flights? Do they use the sun as a compass, are they guided by the earth's magnetic field, do they follow structural features of the landscape such as rivers and mountains, or do they use a combination of these, or perhaps some undiscovered method? Because of their small size (0.5 grams), relatively weak flight, and body temperatures which fluctuate with the air temperature, the monarchs' flight and, therefore, the whole migration should be influenced by the weather conditions. And we need to ask: How do weather patterns and climate affect the migration? Once we have the basic information on the effects of these "physical factors", can we use this information to predict when the migrating butterflies will arrive at particular milestones along their migration routes? Can we predict if the migration will move faster or slower in a given year?
In what direction do migrating Monarchs fly?
It seems logical that migrating Monarchs should move in a southwesterly (225 degrees) direction from where they start. But do they? The butterflies recovered from our tagging program have been found in directions ranging from southeast (150 degrees) to northwest (300 degrees) from the point of initial capture and tagging. Even if the Monarchs are observed sometimes to move in a direct southwest direction, though, the migrants don't have an external map to guide them and they themselves have never been to the roost sites so the question remains: how do they "know" which direction to take?
To obtain an answer to this question, we need to consider several possible means the Monarchs could use to find their way. What is the mechanism of orientation used by Monarch butterflies as they migrate from central and eastern United States and Canada to their overwintering sites in Mexico?
There are several possible ways that Monarchs can orient during their migration flights:
- They could orient based on the position of the sun
- They could orient to the earth's magnetic field
- They could orient using features of the landscape
- They could use a combination of two or even all three of the above - or -
- Perhaps they could use some feature of the environment we are not yet aware of
Each of these possible explanations can be considered an alternative hypothesis. And each hypothesis can yield a set of unique predictions. Testing each hypothesis and distinguishing among them will require experiments that, first, identify differences in their predictions and, second, look at the behavior of the experimentally manipulated Monarchs to see which if any of the predictions best matches the behavior.
Before we design experiments to test each alternative hypothesis, we need more information (data) on Monarch navigation in the field. Here is where we need your help: By making systematic records of the directions Monarchs take on their migratory flights, you can help us find some clues to distinguish between the possible answers to this Monarch mystery. Strictly speaking, making natural history observations in the field is different from conducting an experiment since making field observations usually does not involve manipulating any variable. Most of the best designed experiments, however, grow directly out of insights attained while making non-manipulative natural history observations. Our expectation is that your observations in the field will eventually lead to the definitive studies on Monarch orientation.
What materials do you need to collect data for this project?
If you wish to collect data for this project, you will need the following field supplies: compass, clipboard, pencil, and paper.
What other resources will you need?
You will need information on wind speed, wind direction, air temperatures, and cloud conditions for the day or period during which you make observations. This weather information can be obtained either by personal observation and measurement or by calling the local weather station.
Where should you go to collect flight direction data?
To obtain the most accurate data, observations of flight directions should be made in open areas such as large fields where flight behavior of the butterflies is not modified by hills, buildings, trees, and other such obstacles. Locate a field which migratory Monarchs are passing through. Migratory Monarchs can be identified by the distinct directionality of their flight; they do not mill about randomly. Follow only one butterfly at a time. Select a butterfly that shows clear directionality. Position yourself under the flight path of the butterfly whose direction you will be recording. This will call for you to run around a bit.
What kind of flight direction data should you collect?
There are two kinds of flight information you should try to collect from each butterfly:
1. vanishing bearing and 2. body orientation
Vanishing bearing is the direction in which the butterfly is actually moving through the field toward the horizon. Since this measure is often strongly affected by wind speed and direction, we would also like to know the butterfly's body orientation, that is, which way the butterfly's body is actually pointing.
How do you measure vanishing bearing and body orientation?
To measure a vanishing bearing, you watch a migrating butterfly until it "vanishes" from view and record the compass bearing in which the butterfly disappeared. Here's a trick to give yourself more time to accurately measure the direction the butterfly was moving: after you've run to intersect a butterfly's flight path and as you watch that butterfly flying directly away from you, sight on an immobile object on the horizon like a tree, hill, or pole, that corresponds to the compass bearing of the butterfly. Then, although the butterfly itself will disappear in a few moments, you'll have plenty of time to accurately measure the compass bearing (see notes on using compass below) of the object you've sighted, and can record this compass direction. To measure body orientation, you will need to be fairly close to the butterfly to be able to tell which direction the head is pointing. Two notes about body orientation measurements: 1) since this measurement is less affected by wind direction it can sometimes be in a very different direction from the vanishing bearing and 2) although it is helpful to have both vanishing bearing and body orientation measurements on each subject, it is sometimes impossible to get body orientation data. That's okay. It's better to have missing data than inaccurate data. Always remember to record the time of day that corresponds to each observation as well.
What if the butterflies are not actually migrating while you are outside waiting to collect data?
If the butterflies are not moving in a distinctly directional manner, record the date, the temperature and other weather information, anyway. We need to learn more about the time of day and physical conditions that restrict or favor migratory behavior.
How do you measure compass directions?
To measure directions using a compass and to report your findings you must adhere to certain conventions. 1. Directions are reported in degrees from 0 to 360. 2. North is indicated by 0 degrees, east by 90 degrees, south by 180 degrees and west by 270 degrees. Set the compass due north and determine the angle away from due north that the butterfly is flying. For example, a butterfly heading due south would be flying in a direction 180 degrees from due north. If the butterfly were flying west of due south, we would report some angle greater than 180 degrees, e.g. 20 degrees west of due south would be 200 degrees from the north.
What's the most effective way to show vanishing bearing and body orientation data?
Vanishing bearing and body orientation data can be represented as dots along the outside edge of an empty circle. By convention, the top of the circle (the "twelve o'clock" position) is marked North or 0 degrees. So, a Monarch vanishing while flying due south (180 degrees) would be represented by a dot directly opposite the top of the circle, in the "six o'clock" position. Three Monarchs flying south, then, would be represented as three dots under the circle, one directly beneath the other. An additional Monarch flying to the southeast (135 degrees) would appear in the "4:30" position. And so on. This type of a graph will give you a clear visual representation of your findings that is often more telling than the tables used to collect the data initially.
How many Monarchs should you watch?
Since this exercise gets easier with experience, the first few butterflies that anyone follows should be considered "just practice". After that, during each observation period, every individual or group should attempt to obtain vanishing bearings for at least 20 Monarchs. The number of these butterflies that will also yield body orientation data is often less than 20 since body orientation is a more difficult measurement to make accurately. Disappointing as it might be, no matter how good you get at following Monarchs, not every butterfly will yield usable information. Sometimes that's because of the observer's location relative to the butterfly, but often the butterfly itself will stop to feed or start spiraling upward or do something else. This is normal and might be interesting to keep track of.
When and for how long should you watch the Monarchs?
Because it is possible that the direction of flight changes throughout the day, the time of each observation should be recorded. The observation periods we suggest are:
- 10 am - 12 NOON
- 12 NOON - 2 pm
- 2 pm - 4 pm
Most people will find it convenient to make observations in the afternoon, but some of the most interesting observations can be made in the morning hours. To obtain a thorough understanding of the pattern of movement, observations should be made on several days at different times and in different weather conditions. After several periods or days of observations, the data can be summarized for each observation period. The mean, or average vanishing bearing should be calculated for each period. Differences between periods should be examined along with the corresponding weather conditions for each to determine if environmental factors might influence flight direction.
What do you do with these data?
Once you have summarized your observations, and if you are cooperating in a group, you can share or discuss your results with other members of your group. We would appreciate a copy of your results so that we may compare them with others we have gathered from around the country. We will supply all observers and cooperators with a summary of observations made by other participants.
Where can you find out more about Monarch directionality?
For more information, interested readers might consult the following experimental and analytical papers. You can find these articles and others devoted to the subject in college and university libraries.
- Calvert, W. H. (1996) Miracle of the Monarchs. Texas Parks and Wildlife. October: 24-29.
- Gibo, D.L. (1981) Altitudes attained by migrating Danaus plexippus (Lepidoptera: Danaidae), as reported by glider pilots. Can. J. Zool. 59:571-572.
- Gibo, D.L. (1986) Flight strategies of migrating Monarch butterflies (Danaus plexippus L.) in southern Ontario. In Insect Flight: Dispersal and Migration, ed. W. Danthanarayana, 172-184. Berlin and Heidelberg; Springer-Verlag.
- Gibo, D.L. (1979) Soaring flight of Monarch butterflies, Danaus plexippus (Lepidoptera: Danaidae), during the late summer migration in southern Ontario. Can. J. Zool. 57:1393-1401.
- Schmidt-Koenig, K. (1993) Orientation of autumn migration in the Monarch butterfly. In Biology and Conservation of the Monarch Butterfly, eds. S.B. Malcolm and M.P. Zalucki, 275-283. Los Angeles, CA: Natural History Museum of Los Angeles.
- Urquhart, F. A. and N. R. Urquhart (1978) Autumnal migration routes of the eastern population of the Monarch butterfly (Danaus p. plexippus L., Danaidae: Lepidoptera) in North America to the overwintering site in the neo-volcanic plateau of Mexico. Can. J. Zool., 56: 1759-1764.
- Urquhart, F. A. and N. R. Urquhart (1978) Migrations of the eastern population of the Monarch butterfly in North America to the overwintering site in the neo-volcanic plateau of Mexico. Atlanta, Munnerstadt 9: 133-139.
NOTE: If you wish to determine magnetic declination on your own, obtain a USGS topographic map. You can get these maps from your local public library and city offices, or the ASC (Agricultural Stabilization and Conservation). At the bottom of these maps in the margin, you will find a figure illustrating the difference between true north and magnetic north. Your compass "reads" magnetic north (the magnetic north pole) which is not necessarily the same as true north because of constant changes in the earth's magnetic fields. So, adjusting for local declination tells you what your bearing is according to true north. Adjust your compass readings by either adding or subtracting the declination value for your area.
Thanks for your interest and assistance in this project! Please send us your results along with any questions or suggestions you may have so that we can improve this activity.