Flight Behavior I:
How To Estimate The Altitude Of Migrating Butterflies
The estimated altitude should be recorded as meters above the ground. Always record
the highest estimated altitude for each observation. This procedure compensates, in part,
for the dominant role of wind in determining vanishing bearings. Wind velocity increases
rapidly with increasing altitude above the ground and disproportionately affects the
displacement of the butterflies as they climb into faster moving layers of air. Because
Estimates of altitude are always rough approximations, the data are pooled into into
relatively broad categories for statistical analysis. Consequently, it is unnecessary to
be over concerned about whether a butterfly is at 25, 30, or 35 m, above the ground. Just
make your best estimate guess and move on to the next data entry. One easy method of
recording estimated altitudes is to write the value for in the box labelled 'Est.
altitude', then check off the appropriate box for altitude category (e.g. >3 to
10 m) later when going over the data. This method probably results in fewer mistakes than
the alternative of estimating the altitude, immediately converting the estimate to a
category value, and then checking it off .
There are several methods of estimating altitude, ranging from the simple (eyeball) to the
elaborate (radar). The simple method depends upon the relative size of the butterfly.
First, you have to calibrate your vision by observing the butterflies that pass by structures of known
height. For example, single story, flat roofed, buildings are about 3 m high, power lines
along highways are about 10 m high, and high tension transmission towers are about 30 m
high. Another method is place a preserved specimen of a butterfly (or a life-size
photograph, or a suitable paper cut-out) against a dull white background (simulating
clouds), and setting it at distances ranging from 10 to 300 m and examining it with
unaided vision and , when appropriate, with your binoculars. I suggest trying distances of
10, 30, 100, and 300 m. Within a short time you will get a good appreciation of their
apparent size at different distances. Since more than one butterfly is likely to be
encountered soaring (e.g. monarchs, red admirals, mourning cloaks, and buckeyes),
it is a good idea to practice with representatives of each species. Another method of
achieve greater accuracy in estimating altitude for butterflies flying near the ground, is
to set up a pair of 3 m high flagpoles at the field site. The flagpoles should be easy to
take down and move and should be set about 10 m apart and perpendicular to the wind. They
should have conspicuous marks at 1 m intervals to provide convenient reference points.
Placing ribbons at the top of each flagpole to indicate shifts in wind direction is also
helpful. Good estimates of altitude are possible for migrants that pass between, or near
the flagpoles. Enthusiasts could even rig a camera to record each passing migrant.
Surprisingly, none of the migratory species of butterflies in North America, not even the
monarch, have been systematically studied in this fashion.