Rose map above (Figure 1) shows flight directions of Red
Admirals in northern Italy during a fall migration (Benvenuti et
al., 1994). Procedures described here are based upon the methods
of Benvenuti et al. (1994, 1996) and Schmidt-Koenig
What equipment do I need to observe migrating butterflies?
Basic equipment includes a pen or pencil, a notebook or tape
recorder, a compass, an accurate watch, and binoculars. If you can
find binoculars with a built-in compass, these can be useful for
observing any side-to-side oscillation in flight direction. You will
also need information on the weather conditions, including air
temperature, wind velocity, wind direction, and either cloud
conditions (type of clouds and percent of sky covered) or solar
radiation. The local weather station is a good source of hourly
weather data. You may want to record weather conditions yourself,
especially if you are interested in the effects of shorter-term
changes in the weather. You can find more information on recording
detailed weather data in the Notes on Observing Methods later in this
Where should I go to watch migrating butterflies?
The best sites for observing migrating Vanessa butterflies are large open areas (Fig. 2) as far as possible from trees or buildings or from landforms such as hills, ridges, mountains, ravines, and arroyos, which may cause butterflies to deviate from their prevailing flight direction.** Find an open area through which Red Admirals or Painted Ladies are migrating and choose a spot from which you can easily observe them.
Figure 2. Migration observing site in Ames, Iowa, summer 2001. This view is toward the north. From this location we observed the great 2001 Red Admiral northward spring migrations.
How should I record the flight direction of each butterfly?
Choose butterflies which are flying in a consistent direction
rather than just circling or moving randomly about.*** If a butterfly
passes directly over you, follow it with binoculars until it vanishes
from sight. Then record its bearing with the compass.**** Set the
compass due north and determine to the nearest degree the angle from
due north that the butterfly is flying. 0 degrees is north; 90
degrees, east; 180 degrees, south; 270 degrees, west, etc. Whenever a
butterfly does not pass overhead, but close enough to you, run as
fast as you can to the point where the butterfly was first spotted,
then record its vanishing bearing in the same way. This procedure
minimizes parallax bias which can result from trying to observe from
off to one side. If you have trouble accurately sighting the
direction toward which a butterfly is going, locate a distant object
in the same direction and point the compass at it instead. If you are
working on sloping areas or on sites with loose or stony ground,
avoid running across them and follow only butterflies which you can
How should I record the time of each observation?
Because previous observers have found migration direction to
change with time of day (Baker, 1978, p. 434) or with the temperature
at particular times of day (Benvenuti et al., 1996), the time
of each observation should be recorded, at least to within 5 minutes.
To the nearest minute is better. You should set your watch
accurately, at least to the nearest minute. For some research, to the
nearest second may be better. Accurate time signals are broadcast
from the U.S. National Institute of Standards and Technology radio
station WWV, Fort Collins, CO. Carrier frequencies are 2.5, 5, 10,
15, and 20 MHz. WWV gives Coordinated Universal Time (UTC). To
convert to U.S. time zones, subtract 5 hours for Eastern Standard
Time, 6 for Central, 7 for Mountain, 8 for Pacific, 10 for
Alaska-Hawaiian. If you are observing when Daylight Saving Time is in
effect, be sure to state whether you are recording your observations
in Daylight Saving Time or standard time. States such as Arizona and
Hawaii use standard time year-round.
How long is a typical observing period? What are the best times of day to observe migrating butterflies?
Observing sessions typically last from one up to six hours (if you
are especially patient or enthusiastic). Because butterflies need
sunshine to stay warm enough to fly vigorously, migration is best
observed several hours either side of early afternoon. Chip Taylor of
Watch recommends two-hour periods at these times: 10 a.m.-12 n.,
12 n.-2 p.m., and 2 -4 p.m. Do not observe for such a long time that
you get tired or lose interest. Doing so can introduce bias into the
data. Longer observing periods are best divided among several
observers, especially if observing continues for many days.
Is it better to observe butterflies over an extended period of a few days to several weeks?
Yes. To thoroughly understand the pattern of migration, you should
observe butterflies for at least several days at different times and
in different weather conditions. Abundance, flight speed, and flight
direction of migrating butterflies often depend on both time of day
and weather conditions such as temperature, wind speed and direction,
and cloud cover. On cooler days, butterflies may be able to migrate
only around mid-day. V. atalanta butterflies of some
populations may change their flight direction as the sun' s azimuth
(compass bearing) changes (Baker, 1978, p. 434). Shareware and
freeware programs such as SunPo and Home
Planet can be used to calculate solar azimuth for such
How precisely do I need to know where my observing site is located?
You'll need to know your location fairly accurately. You can
locate your site precisely with USGS topographical maps. You can
often find these at a local library or municipal government offices.
If at all possible, locate your site to the nearest mile or nearest
minute of arc in latitude and longitude. You can also give your
location with respect to known landmarks or numbered north-south and
east-west roads, or as section, range, and township numbers within a
county. Best of all is a GPS (Global Positioning System) indicator.
With one of these, you can locate your site to arc-second (or better)
accuracy. You can get similar accuracy from a mapping site such as
How do I plot and analyze flight direction data?
Flight direction data are typically plotted on a radial "rose graph", with lines or arrows extending outward from the center of the graph (see top of page). The angular position of the line or arrow corresponds to flight direction. The length of each line or arrow is proportional to the number of butterflies seen flying in that direction. If there is a preferred flight direction, the arrows pointing nearer to that direction will be longer. If you understand statistics, you can use circular statistics (Mardia, 1972; Batschelet, 1981) to determine the mean flight direction, the variance in flight direction, and whether butterflies tend to fly significantly more often toward the mean flight direction. You can also find out if the mean flight directions in two or more sets of data taken at different times or in different conditions are significantly different.
Software for computing circular statistics is also available. The program Oriana calculates statistical parameters and plots circular graphs in several different formats.
What else besides flight direction can I record while observing migrating butterflies?
Some other possible observations include hourly and daily counts of migrants and flight speed.
How do I count the number of butterflies passing through my observing site?
You can record the number of Red Admirals or Painted Ladies you sighted each hour during the observing session. If too many butterflies are passing by, however, some may pass unseen while you are measuring the flight directions of others. If you want to do both at once, it is better to be one of a pair or group of observers. Only butterflies which are within 30 to 50 meters away (depending upon the color of the background) will typically be visible. For this reason, it is a good idea to define a line of known length, perhaps 30 meters or 100 feet long, that is perpendicular to the general direction of flight, then record the number of butterflies crossing this line per minute, per hour, or per some other interval. Because butterfly migration rates often vary widely, from a few to thousands of individuals per hour, the best time interval to report results may likewise vary. For example, Derham Giuliani, who observes enormous Painted Lady migrations through California, typically records his results as number of butterflies/50 feet/5 minutes.
If several observers count butterflies in the same place at the
same time, they should stay at least 100 m apart so that no butterfly
will be counted by more than one person. You should observe long
enough to get enough data to calculate an accurate hourly rate. This
is especially true if butterflies are arriving in bunches separated
by intervals when none are passing by. You can also accumulate your
total counts for each day over a period of days to estimate the
start, end, peak day, and duration of the migration. If you are doing
so, I recommend taking data during the same period of time each
How do I measure the flight speed of a migrating butterfly?
Flight speed can be determined by measuring the time required for single butterflies to move between two poles placed at least 40 m apart along the prevailing flight direction. For precise results, the butterflies chosen for observation should be those flying as near and as parallel to both poles as possible.
What is a good way to plot hourly and daily counts and flight
speed of migrants?
Such observations can be plotted as bar charts. Number of butterflies seen each hour or day is plotted on the vertical axis and hour or day on the horizontal axis. Distribution of flight speeds can be plotted with speed on the horizontal axis and the number of butterflies flying at a particular speed on the vertical axis. You can also try plotting counts or mean flight speeds against wind direction, wind speed, temperature, time of day, etc.
Additional Notes on Methods
*How should I record weather conditions?
If you measure weather conditions yourself, you should do so at least hourly, and perhaps more often when conditions change frequently. You can record temperature with an ordinary thermometer. This should be shaded (a piece of cardboard or a toilet tissue core painted gloss white surrounding the thermometer bulb is good) and mounted on a pole at a consistent height above the ground out in the open. This can be one of the standard heights such as 5 feet, 1.6 m, or 2 m used by meteorological stations, or you can estimate and use the average altitude of the migrating butterflies. More frequent and precise measurements can be obtained with more precise thermometers, or a shaded thermocouple connected to a digital thermometer or automated datalogger. When I use a datalogger, I usually take readings once per second and average these once per minute.
A simple homemade wind vane may show the wind direction accurately enough for some purposes; wind vanes of the type used with a datalogger are much more precise. Wind velocity can be roughly estimated by observing the movement of various objects in the wind (Beaufort scale); such estimates should be compared with wind velocity measured at a local weather station. Wind velocity can be measured more precisely with a hand-held or pole-mounted anemometer. Solar radiation can be measured with a pyranometer (watts/square meter) or a light meter. This should be either kept level or pointed directly at the sun during each reading. An alternate method is to note the type and extent of cloud cover present (e.g., clear, cirroform, cumuliform, overcast) during the observing period. On days with broken cloudiness, the times when the sun goes in and comes out can be recorded. Some correlation of the sunny periods with increased migration rate may be evident.
**How might objects and landforms affect migratory movements of butterflies?
Red Admirals and Painted Ladies often fly toward hilltops and along ridgetops, trails, cliffs, and similar visually prominent features. If you are specifically interested in finding out how butterflies fly around or along such features, this too can be an interesting study. However, such data should not be lumped together with observations of migration direction in open areas.
***Why are the movements of butterflies sometimes
non-directional, even on days when they have been migrating?
If you see Vanessa butterflies circling or milling about during the late afternoon, they may be starting perching or territorial behavior, and they will stop migrating for the rest of the day. Vanessa butterflies seen in early to mid-morning may have just left their treetop roosts and may bask and circle to warm themselves up.
****Should I record flight headings as geographical or magnetic
Magnetic north as shown by a compass varies from true north by several degrees in most places. You should state whether your angular measurements are recorded with reference to magnetic or true north. The difference between the two is called magnetic declination. To find magnetic declination for your area, you'll need to know two things: your location and the date when you observed. You'll need the date because magnetic declination changes gradually over time as the earth's magnetic field varies. One site where you can find this information is the Estimated Value of Magnetic Declination, 1900-2010, maintained by the (U.S.) National Geophysical Data Center (NGDC).
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This page was updated on May 11, 2009.
Baker, R.R. (1978). The Evolutionary Ecology of Animal Migration.
Hodder and Stoughton, London, 1012 pp.
Batchelet, E. (1981). Circular Statistics in Biology. Academic Press, London.
Benvenuti, S, Dall'antonia, P, and P. Ioalè. (1994). Migration pattern of the red admiral, Vanessa atalanta L. (Lepidoptera, Nymphalidae), in Italy. Bollettino di Zoologia 61: 343-351.
Benvenuti, S., Dall'antonia, P. and P. Ioalè. (1996). Directional preferences in the autumn migration of the Red Admiral (Vanessa atalanta). Ethology 102: 177-186.
Mardia, K.V. (1972). Statistics of Directional Data. Academic Press, London and New York.
Schmidt-Koenig, K. (1979). Directions of migrating monarch butterflies (Danaus plexippus; Danaidae; Lepidoptera) in some parts of the eastern United States. Behav. Process. 4: 73-78.
Scott, J.A. (1992). Direction of spring migration of Vanessa cardui (Nymphalidae) in Colorado. Journal of Research on the Lepidoptera 31: 16-23.