Tactics and Vectors 98/99
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What To Look For In A Field Site

farm in flatlands with surrounding field of cornLocating a good field site is usually something of an adventure. Key considerations include accessibility and suitability for making observations. Although parks and playing fields in built up areas are possible sites, the best sites are in rural areas or range land. The ideal site is the middle of a large, flat, open, field, with unrestricted views in all directions. In the real world, most sites are likely to be reasonably large and relatively flat fields with more or less unrestricted views in most directions. Try to avoid areas with with hills, cliffs, ravines, adjacent wood lots, sea shores, lake shores, ponds, buildings, or isolated trees. All of these features are likely to distract or redirect the migrants. In addition, most structures, particularly larger structures, interact with the wind to generate complex patterns of turbulence. Avoid sites with obvious linear features. Migrating butterflies tend to fly parallel to lines of trees, hedge rows, streams, and even drainage ditches. A field site located in the rural setting pictured at right would present several problems. Unless the butterflies are flying 30 m or more above the ground, the buildings, silos, clusters of trees, and the abrupt break between the corn field and pasture, would all present problems in interpreting directional data. If something like the farm site is the only option, then moving further away from the buildings is probably the best compromise. Unfortunately, the ideal site tends to be bland and lonely.

Hayfield with rolls of hay,  and nearby, tree-covered,  hillsThe hayfield to the left is an acceptable field site but there are several problems to consider. Although the trees are probably too far away to distract migrants before you have recorded their vanishing bearings, butterflies seeking roosting sites may head for the woods in late afternoon or if the weather begins to deteriorate. The drainage ditch in the mid ground may pose another problem. Such an obvious linear feature may be followed by the butterflies when they are flying close to the ground, particularly in crosswind conditions. Furthermore, the hills in the background will generate local wind patterns that will affect even butterflies flying more than 30 m above the ground. For example, a wind from behind and to the right of the observer may be channelled along the face of the hills, altering the direction of high flying butterflies compared with those flying over flatter terrain. Finally, turbulence generated by the rolls of hay in winds could also affect the butterflies behaviour. In a situation like this, the best solution would probably be to find a field further from the hills, with no drainage ditches, and no rolls of hay.

fields in flower with fenceThe attractive field on the right is not acceptable as a field site because of the large areas of flowers. Although visibility is excellent, and it is highly unlikely that the butterflies would even notice the fence, the flowers would be a major distraction. As long as the butterflies are flying well above the ground (e.g. > 100 m)in tailwind conditions, we can probably assume that they will ignore large fields of flowers. On the other hand, when the butterflies are flying within a few meters of the ground, particularly if there is a light headwind, it is impossible to determine if they are migrating, or just following the scent of a preferred species of flower. Migrants flying over fields of flowers at intermediate at intermediate altitudes may or may not be distracted. There simply are no data. However, I expect that the stimulus of large fields of flowers would cause at least some individuals to descend and begin nectaring, particularly in the late afternoon as the days migration begins to wind down. Since there also appears to be large fields of grass in the vicinity, the best plan would be to locate the site in one of these areas. Finally, because no single field site is ideal, there is always the temptation to monitor several sites, each with a different combination of advantages and problems. Unless your research program can have two or more observers monitoring different sites simultaneously, observing the behaviour of migrating butterflies at one site under a wide range of conditions will generally provide more valuable data than observing them at several sites at different periods of times and under a narrower range of conditions.

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At last - a nearly perfect landscape for studying butterfly migration.  The view to the horizon is essentially unlimited and distractions are minimal.  Only one problem - obtaining permission from the local landholder (pictured above) may be a challenge.