Tactics and Vectors 98/99
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Table of  Contents

Great Circle Hypotheis  

Magnetoclinic Hypothesis

Magnetic-Latitude Hypothesis

Compass Bearings Hypothesis

Suns' Azimuth Hypothesis

Expansion-Contraction Hypothesis

Always Advance Hypothesis

Never Go Back Hypothesis



Flight Behavior V: Vanishing Bearings And Headings



How To Take Vanishing Bearings and Headings (Body Orientations) of Migrating Butterflies

A quick course in using a compass

Buy a good orienteering (map reading) compass - the clear plastic type with the compass needle set in a clear plastic capsule filled with a transparent fluid. The capsule is set in a clear plastic, rectangular, base plate and is free to rotate. You will find this type of compass at most sporting goods stores. The large sports warehouse stores will have perfectly serviceable models and stores specialising in camping, hiking, orienteering, etc. will have more expensive models. If you have not used a compass before, note that the scale around the rim is marked in degrees and shows the cardinal compass directions N, E, S, and W. There is a large arrow etched at one end of the rectangle base plate that is used to sight on objects, or along flight paths. The north-seeking end of the compass needle is conspicuously marked in some fashion and usually painted red. Finally, note that the fluid-filled capsule also has a red (usually) arrow etched on the bottom that points to N (360) on the scale. Taking a magnetic bearing with a map reading compass is easy.

  • First: hold the compass level and point the arrow on the base at some object in the distance and let the compass needle come to rest. Unless the bearing of the object is N (mag.), the north-seeking end of the compass needle will not be aligned with the N-pointing red arrow etched in the bottom of the capsule.

  • Second: if the two are not aligned, correct the situation. While continuing to hold the compass level and pointing at the distant object, rotate the capsule until the N-pointing red arrow and the north-seeking end of the compass needle are aligned.

  • Third: read the bearing (mag.) of the object from the point on the degree scale closest to the arrow pointing at the object (i.e. the side away from you). At least one manufacturer (SILVA) makes things easy by having black indicated below the rotating capsule that is in line with the large arrow. They even write 'read bearing here' at this point.

That's all there is to taking a bearing. You may wonder if a more complicated (and bulky) compass would provide greater precision than the plastic orienteering variety. It would, but only if you plan to use surveying techniques, mount the compass on a stable and levelled platform and spend a lot of time taking careful readings. I am prepared to settle for a small increase in variance in exchange for a much larger sample size. In any case, altitude changes, circling flight, and erratic flight introduce far more variance.

Taking Bearings of Butterflies

Vanishing Bearings are taken by observing the butterfly, either with unaided vision, or through binoculars, until it vanishes in the distance. Look at the horizon and pick out a conspicuous object (fence post, bush, rock, etc. along your line of sight. Take your compass, sight along the flight path to the selected object, and record the bearing. This is the vanishing bearing for the butterfly. Only record the vanishing bearing (mag.) on the data form. Do not attempt to convert magnetic bearings to True Bearings in the field. It only causes confusion, particularly after about the third hour standing in the sun. Unless, of course you're entering your data directly into a spreadsheet on your expensive laptop computer that you have already already programmed for the appropriate correction factor. I don't recommend this approach. I use to pack at least three tape recorders for my field trips to Georgia and Texas just be sure of having at least one that still functioned at the end of two weeks.

Taking Headings of Butterflies

Headings, or body orientation are more difficult to observe than vanishing bearings. Until you feel comfortable taking vanishing bearings, it is better to simply note if the body of the butterfly is turned to either the left or right of its flight path. Then record the heading is either R (turned to the right) or L (turned to the left). This is important data because it can some indication of whether or not the butterflies are compensating for crosswinds (i.e. turning their bodies partially into the wind to offset drift - more on this later in the testing hypotheses section). The heading of a butterfly is taken in a similar manner to the vanishing bearing. You sight along the body of the butterfly to a conspicuous object and then record the bearing for the object. This bearing is the heading. There are only two problems, but they're more than enough. First, the butterfly is usually aligned so that you can sight along its body for only a brief moment as it passes by. You have to be quick. Second, although a record of a vanishing bearing without the corresponding heading is valuable data, a record of a heading without the corresponding vanishing bearing is useless. You have to either get both vanishing bearing and heading from the butterfly, or just the vanishing bearing. The method I favor is simple. As the butterfly passes by you quickly point in the direction of the heading and memorise whatever conspicuous object along the line of sight. Then you continue to observe the butterfly until it vanishes. Record the vanishing bearing, then record the bearing for the heading. Obviously, this is a lot easier if you are not using binoculars. If you are using binoculars, you have to point in the direction the butterfly is facing while continuing to observe through the binoculars.. You then glance in the direction you are pointing, memorise a conspicuous object, then return your gaze to the binoculars and continue to follow the butterfly as above. This technique is awkward, but with practice, it works.

Three categories of vanishing bearings

Category I vanishing bearings
Both the vanishing bearing and the heading are aligned. Altitude range is not specified. Category one vanishing bearings are seen only in butterflies exhibiting straight flight and flying either directly upwind, directly downwind, or in calm. It is impossible for the the two bearings to be aligned in any other wind conditions. This is the only situation where we can be reasonably certain that the butterflies are in complete control of their direction. Consequently, category I vanishing bearings are the best data set for calculating the preferred direction of the butterflies. Unfortunately, Category I vanishing bearings cannot be taken for species that migrate by continuos erratic flight.

Category II vanishing bearing
The butterfly is within 3 m of the ground. Heading may or may not be recorded. The two categories overlap for individuals Category I observations of individuals flying within 3 m of the ground. Observations of monarch butterflies show that the mean direction (angle) of vanishing bearings of individuals flying within 3 m of the ground cannot be distinguished from the mean direction of Category I vanishing bearings (Gibo 1986). The two measurements, however, differ greatly in variance. Nevertheless, the butterflies have good control of their courses. Category II vanishing bearings are the second best data set for calculating preferred directions.

Category III vanishing bearings
This last group includes all bearings that are classified as either Category I or Category II. The butterflies are clearly exposed to the wind. The amount of control that they have over their flight direction depends on wind direction, wind velocity, and their flight speed. Flight tactics can have a major impact on the direction of displacement.