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Dear Journey North,
A person with hundreds of trees is in an excellent position to answer
some very basic questions about roost formation. To study this, a person
should note the exact position of the roosts each year, along with daily
observations of:
- Wind direction
- Humidity (dew point is the best measure of this),
- and, if in a drainage channel, the butterflies' position with respect to the sides of the channel.
When watching the roosts form in the evening, one is struck with several things:
- There is active patrolling amongst the canopies of trees. Many butterflies appear to be flying around looking for something.
- Some butterflies land on perches. When another butterfly approaches their position the perched butterflies open their wings, often rather abruptly.
- After this disturbance they may continue to open their wings for a few seconds more,"flashing," usually much more slowly than initially.
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Researchers have considered two possible meanings for this conspicuous behavior:
One is the obvious one: That the perched butterfly is signaling for other butterflies to come join it. But why only signal when another butterfly is approaching? Again the answer may be the apparent one: Why bother to signal when there is no one around?
Still there is another intriguing interpretation. The perched butterfly may be signaling to come join it, but with conditions. The conditions would be to go to the periphery, a position more dangerous than the center. (In any sort of school or aggregation, it is always safer to be in the center. Your predator is likely to strike at the periphery.)
So far no one has figured out a method of teasing out an answer to this question. Maybe you would like to try?
Has
anyone done research into this?
One year along the migration route in Mexico's Sierra Madre Oriental,
we encountered an immense flower field with isolated huisache trees in
it. Each evening monarchs that had been nectaring on the flowers would
stop, and fly towards the huisache trees. They would find each other and
form transient, gregarious roosts. One evening we carefully noted the
branch they were on. The following day we cut that branch out of the tree
and tied it in position in another quadrant of the tree. That night we
waited with anticipation. Well the wind had changed. The butterflies did
not go to our special limb, but rather went to the lee side of the tree,
as is their custom.
So what can we conclude from this? Not much. The cut tree limb may have
been a bit drier than the tree limbs attached in the regular manner and
therefore, rejected by the butterflies. But it seems that the dominant
effect was the wind and not some sort of pheromone deposited by butterflies
on the branch the previous night.
Dr. Lincoln Brower and some of his students tried to find an attractant
at a Mexican overwintering colony where there were millions of butterflies.
They found lots of chemicals associated with the fir (oyamel) trees, but
nothing that could be measured as a pheromone.
It seems likely that the butterflies are looking for these things:
- A tree where they can be off the ground and therefore safer.
- A place out of the wind where they will not be blown from their perches.
- Each other, so they can be safer in numbers.
- In dry habitats, they also seek trees in riparian areas where the humidity is high.
Dr. Bill
Calvert
(Meet Dr.
Calvert)
