Calvert's Observation in Mexico:
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.
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.
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). "