Sun and Shade
The shade moved with time. The sun's rays reached the grass in the distant part of the meadow first. The dew has evaporated there. The grass is already dry. The grass in the shade is still wet.

Dr. Calvert's Observation in Mexico:

"In the early mornings, butterflies pour into the fields to drink. I have noticed that they concentrate on the sunny side of a shaded patch. They are clustered in a narrow band only 1-1.5 meters wide.

"They're drinking dew there, just before it evaporates in the strong tropical sun. The monarchs can't seem to function well in the shade, so they move in this narrow band of sunshine where the sun hasn't evaporated the dew yet.

"By noting whether dew or frost is present, we know if this important source of water was available to the monarchs that day. As the dry season progresses, the dry air dries out the forest and the butterflies. The wind, and the lack of moisture in the ambient air, dries them quickly. Dew is often no longer available. The monarchs have to go down to streams (and to nectar, if available). "