Our colleague from NASA, Dan Slayback, has mapped out all the individual
tree positions where we have installed these along literally hundreds
of meters of paths through the forest. Imagine trying to find each
I-Button on a 4 inch long black post attached to the bark of Oyamel
trees. We are able to do this using a compass, a laser rangefinder
and a GPS global positioning unit. Keeping careful notes is essential!
An "I-button" measuring air temperatures.
as we found on this expedition, by the middle of March, the monarchs
are able to obtain substantial nectar. To determine this was a bit
grisly, because we had to operate on the monarchs and determine
how much nectar was in each crop. Many crops were full, but some
had only air in them because the flowers can be so stressed by the
severe dry season that they can not produce significant quantities
of nectar. The butterflies then suck in air that fills their crops
like tiny balloons.
visited the Sierra Chincua colony on Friday 10 March 2006 and found
that it had moved down to lower altitude in a beautiful valley known
as the Arroyo Honda. Huge clusters of monarchs had reformed on the
trees here, presumably because the headwaters of these stream dry
out as the season progresses and it is much more moist lower down
in the valley. Several years ago, Bill Calvert and I found that
the colonies typically move down their arroyos slowly in February
and March, and then roll out of the canyon mouth towards the end
of March. We wanted to collect some of these late spring monarchs
and so returned on Tuesday to the exact spot where they had been.
virtually every single monarch was gone! They had left over the
weekend, probably on Saturday morning. Now we were really worried,
because we thought that all the colonies had broken up and headed
north! However, we went the next day to the Cerro Pelon colony...which
on this separate mountain was gradually moving down the Ojo de Agua
to our relief, we found that many clusters, though thinned out from
their earlier high winter densities, were still present. The colony
was spread out over a larger area and the monarchs seemed agitated
and on the threshold of leaving. Standing quietly in the forest,
we would hear an occasional whispering crescendo......and learned
quickly that what was happening was that one or several clusters
of monarchs suddenly "explode" with butterflies cascading
out into the air like a waterfall flowing sideways!!
believe that these explosions are one of the ways in which the spring
remigration begins....once the clusters break and the individuals
begin flying in the now much warmer weather, many clusters probably
break up in rapid succession, with a building tide of monarchs moving
down hill, out of the valley, and thence northward. This explosion
behavior is absolutely stunning to witness, and if one is lucky,
it is possible to catch the cascading monarchs in a still photo,
or even better, with a video camera.
Another discovery we made was that in the morning when monarchs
gather together and drink in sunny spots where water is flowing
slowly across the ground. Early in the day they all align themselves
and open their wings so that the sun shines directly on their thoraces.
Several years ago, we determined that a monarch's wing muscles have
to be about 15 degrees centigrade before the butterfly can fly.
We measured the temperature of the water at these seeps and found
that it was about ten degrees centigrade. Apparently, if the drinking
monarchs were not sun basking, the ingested water would cool the
flight muscles to below flight threshold and the butterflies could
then get trapped on the ground!
I have been going to these overwintering colonies almost every year
since January 1977. On each visit, I discover something new about
this wonderful and fascinating creature.
Sweet Briar College
Sweet Briar VA.
Crop of a butterfly, as illustrated in 1880 by Burgess.