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Table of  Contents

Magnetoclinic Hypothesis

Magnetic-Latitude Hypothesis

Compass Bearings Hypothesis

Suns' Azimuth Hypothesis

Expansion-Contraction 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.