Several thousand monarch butterflies roosting overnight is an unbelievable sight, surely one of nature's greatest marvels. It is most likely about now but not every year -- seldom in fact, because there must be a coincidence of large migratory population, a sudden switch from a north to a south wind, flower food, and mature broadleaved roosting trees. At about 4:00 p.m., if it is cloudy, cool, or windy, or later until dusk (7:15 p.m.) under clear, calm skies and falling temperatures, monarchs assemble in pompom-like clusters beneath leaves in the lower canopy. Next morning, around 8:00 -9:30 a.m., as the sun strikes them and the air warms toward sixty degrees, the pompoms become confetti blowing in the wind.

The fall migration of monarchs peaks this week, although individuals will drift by through November, nearly all preceding the first freeze (average November 20). Even the last ones are usually a few days ahead of a mostly leafless tree canopy (average November 28). Obviously, roosting monarchs avoid exposure to the night's lowest temperatures. Right now the nightly minimum averages four and a half degrees higher below tree canopy level than in the open. After deciduous trees are leafless, there is only one degree of difference. Thus does the fall passage of monarchs avoid harsh conditions, another instructive example of fine environmental tuning.