monarchs

Paul Cherubini paulcher at concentric.net
Sun Sep 27 17:45:28 EDT 1998


David Albaugh wrote:

. First off, they all congregate on the north
> sides of the trees - is this a normal pattern?

They typically cluster on the leeward side of a group of trees. When the
wind shifts direction, the clusters will reform on the new leeward side.

 From what I have seen,
> instead of hundreds of monarchs gathering on one tree (which I observed
> at the same location 20 years ago) it now seems that 2-3 dozen gather
> per tree scattered over a wide area - is this the norm?

Regional numbers are down this year in the eastern half of the nation as
compared to the previous two years.  This results in smaller clusters
forming over a wider range of trees. High temperatures also result in a
scattering of clusters. Cold temperatures result in fewer, but larger
sized clusters.

 I have also
> observed monarchs flying about trying to locate their nightly rest spot
> - what makes them decide on the particular tree? 

It depends on temperature and wind conditions. On a hot day, most of the
butterflies seek deep shade.  In any stand of trees there are going to
be certain branches that offer a combination of deep shade and wind
protection.  Most end up finding the same branches because as
individuals, they are all individually seeking the same optimal
combination of shade and wind protection.  When flying or resting close
to one another, autumnal monarchs are socially "gregarious" in that they
seem to want to roost next to one another. They are not interested in
mating as a rule.  

On a cold day, they may seek a sunnier branch located higher in the
canopy, but still on the leeward side.

Do they actually see
> other monarchs already there (and if so, what makes the 1st monarch go
> there)

Yes, it's all visual as far as we know. No aggregation pheromones have
been identified.

 Why is it that night after night are they not only on the same
> trees but they are on the same spot on the same trees? 

See above. Roosting positions will change if the wind and temperatures
change and as the season progresses.

Finally, during
> the summer months where do monarchs spend their evenings - do they go to
> these same trees or is the tree congregation only a migration thing?

Summer monarchs are not socially gregarious. The males have an intense
sex drive and will chase and try to mate with anything floating through
the air or any butterfly sized object they see resting in a tree.  At
night (with rare exceptions) both males and females roost as
individuals. Multiply mated females may roost deep inside a tree canopy
to help avoid detection by the males.

Paul Cherubini, El Dorado, California
paulcher at concentric.net


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