Monarch
butterflies and mountain ranges. (adapted from tvectors)
Cherubini, Paul. March 31, 1998.
In a recent posting (3-27-98) David Gibo provided a facinating account of how migrating
monarch butterflies may use thermals in combination with upslope winds to rather
effortlessly gain the 5,000 to 10,000 feet of altitude necessary to cross the High Sierra
Nevada Mountain Range in late summer. This was a real eye opener for me as I did not
realize that thermals and a strong lapse rate were associated with the fair weather high
pressure systems that are sometimes parked over the Great Basin in late summer.
But is this the actual way monarchs routinely get over the Sierra's? Do the butterflies
sometimes "waste" time waiting for the right thermals and upslope winds to
develope before venturing over the high mountains? Or do the butterflies also regularly
cross the mountains using energy intensive powered flight when headwind conditions exist?
I have some evidence to support both scenarios. Several years ago around Nov. 5 I captured
a few hundred monarchs from an overwintering colony along the California coast, then drove
inland to Buttonwillow, Calif. (near Bakersfield, Calif.) where I tagged and released them
at dusk. Because the release was done at sunset, the butterflies immediately sought refuge
in nearby trees for the night. The following day I drove over to the California coast to
the Santa Barbara region where I searched the overwintering colonies for tagged
individuals using binoculars and a spotting scope. Strong offshore Santa Ana winds were
blowing at the time in this region. At about 1:00pm in the afternoon, I spotted one of the
Buttonwillow butterflies in an overwintering colony at Gaviota State Park, a little west
of Santa Barbara. I was amazed! This tagged butterfly had managed to fly roughly 75 miles
in about half a day! And it crossed the fairly formidable Coast Range mountains in the
process. To accomplish this, I assumed the butterflies had soared on the gusty offshore
Santa Ana winds, much as David Gibo has suggested in his wind and thermal assisted Sierra
crossing scenario.
However, I have also watched monarchs using powered flight in the Sierra Nevada mountains
to maintain a southwesterly flight direction during headwind conditions. When I was a
teenager, family outings often took us to the northeast shore of Lake Tahoe during the
Labor Day weekend in early September. At that time of the year, the usual weather routine
was clear and calm conditions in the morning and gusty westerly onshore winds during the
afternoon hours. Almost every day from the beach I would see a total of 2-3 monarchs per
day during the afternoon hours, launching southwest out across the Lake almost directly
into the brisk westerly winds. The butterflies flew extremely low, almost touching the
water and managed to maintain a fairly brisk pace despite the formidable headwind. Since
westerly breezes are common at virtually all elevations and on both the east and west
slopes of the Sierra during the afternoon hours in late summer, I believe the butterflies
I saw heading SW across Lake Tahoe continued up and over the Sierra crest, bucking the
wind all the way.
In conclusion, my current hypothesis is that on a typical September day in the Sierra, the
butterflies soar high in the air during the morning hours on thermals and upslope winds,
as David Gibo suggests. In the afternoon, however, the butterflies may frequently
encounter westerly headwinds and will switch to low altitude powered flight to continue on
a southwesterly track.
In California, migrants in September face other interesting geographioal challenges as
well. Consider how monarch would handle the problem of crossing the scorching sand dunes
of Death Valley and the Mojave Desert at a time of the year when midday temperatures are
routinely around 110 degrees F. Durham Giuliani of nearby Big Pine, Calif. and I talked to
a Death Valley park ranger that has an interest in monarchs. She reported that around the
third week of September, when the peak monarch movement comes through, the butterflies
handle the midday heat by clustering by the hundreds in the skimpy shade provided by trees
planted near the park admistrative buildings. So apparently the butterflies migrate across
Death Valley only during the early morning or early evening hours, at least when headwind
conditions prevail. However, when easterly or northerly tail winds are present, presumably
the butterflies soar on thermals a mile high or more in the air where temperatures would
be moderate even in midday and in this way make quick progress accross the Mojave desert.
Paul Cherubini, El Dorado, California (near Sacramento) email: paulcher@concentric.net
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