Flight Behavior
IV: Flight Path in the Vertical Plane
Characteristics of Level, Climbing, and
Descending Flight Paths
After wading through the ambiguities in attempting to classify flight behaviours, type II and III, classifying the flight path according to whether the butterflies are staying level, climbing, or descending, should be relatively easy, and it is, sort of. Level flight means just what is says - butterflies in level flight continue on without loss or gain of altitude. It seems simple, but we are immediately presented with problems when we attempt to classify the flight paths of cloudless sulphurs, Gulf fritillaries, buckeyes, and other species that often engage in long periods of erratic flight. Since the migrants are constantly bobbing up and down, all that that can be done is to estimate the average altitude of the flight path and judge whether this remains constant, increases, or decreases. Altitude of the migrants is no help. Migrating engaged in erratic flight may fly within several meters of the ground (e.g. cloudless sulphurs) or may fly higher than 30 meters (e.g. buckeyes).
Butterflies that are Climbing are, on average, steadily gaining altitude, while Descending butterflies are steadily losing altitude. The migrants may be bouncing around if they are engaging in erratic flight, or if they have encountered turbulence. In both cases, the mean altitude of the migrants is either steadily increasing, or steadily decreasing. Classifying the vertical flight paths of monarch butterflies is relatively easy because the butterflies seldom engage in erratic flight. There are only two complications - turbulence and altitude. Turbulence has been discussed under the heading of erratic flight. Accuracy in judging the vertical flight path declines when viewing high flying individuals. When a butterfly passes overhead at an altitude of 100 meters or greater, it is usually impossible to tell whether it is flying level, slowly gaining altitude, or slowly descending. Parallax error limits our ability to distinguish level flight from slow rates of change in altitude for butterflies passing overhead at moderate ground speeds. The situation is much worst when the migrants are flying with strong tailwinds and are zipping overhead at a high groundspeed. You can only be sure that an individual is descending when it has its wings half folded and is dropping like a rock. Similarly, you can only be sure that butterfly is climbing if is a strong thermals and rocketing upward. However, if reasonable numbers are passing overhead, and none seem to be either dropping out or vanishing upward, it is reasonable to assume that they are all in level flight.
Now for a quick quiz. Assume your field site is on the top of a hill and two butterflies fly by, the first of which remains at the same altitude with respect to mean sea level while the other flies down the hill and remains exactly 2 meters above the ground. How do you classify the two flight paths? Is the first butterfly in level or climbing flight? Is the second in level or descending flight? Answer: the first butterfly is in level flight (no change in potential energy) while the second is descending (potential energy is decreasing). But who cares? You will never have to wrestle with this problem because you followed recommended procedures and located your field site on level ground, well away from hills or any other confounding variables.