Dear Journey North,
People often ask why monarchs roost where they do during fall
migration. Do they rest at the same sites every fall? Many people
swear that they do but the answer is clearly no. Their use of
a particular roost site depends on several factors. The list below
may not be complete. And even the 'perfect' roost site —
with all or many of these characteristics — will not always
have monarchs roosting during the migration. There are simply
too many variables that change from year to year.
are factors to consider:
1) Where were the monarchs before they
This factor is the most important. Monarchs travel each day with
the winds. The migration is often synchronized in the form of
waves of butterflies that pass over a particular point. If the
monarchs' starting point is a distance upwind from your position,
such that the wind has an appropriate velocity and duration to
carry them to you, butterflies will "fall out" (to borrow
a birding term) and you will be graced, and perhaps dazzled, by
2) Are you on a principal flyway?
If so, your chances of seeing roost trees are much improved. A
principal flyway is very likely defined by its geographic location
along a path from a principal breeding area to the Sierra Madre
Oriental Mountains of Mexico. (This latter mountain range seems
to focus the migration in Mexico, and direct it towards the overwintering
sites located in the Transvolcanic Belt of central Mexico.)
3) Are nectar sources close by?
Nearby flower fields definitely improve your chances of seeing
roosts. Often monarchs will come down in the afternoons to feed.
As the sun sets, they stop feeding and fly to nearby trees where
they commence a search for eachother to form "transient night-time
4) Does the site provide protection from
Monarchs will roost downwind of the wind direction when they form
their roost. Sometime the wind reverses at night and they are
caught in positions exposed to the wind.
5) Are you in a stream valley or depression?
Especially in dry climates, monarchs seem to be attracted to cool,
moist areas. When winds are coming from the south, they seek out
these "riparian" areas to relax and hang out until the
winds turn around. Once they do turn around, the monarchs are
gone in a flash. Also in dry climates they prefer protected shelters
afforded by overhanging trees. In the main flyway, an arc of oak
or pecan trees over a stream channel will almost always yield
a roost of migrants.
6) Do you have an oasis of trees?
In an area otherwise clear of trees, monarchs are often attracted
to oases of isolated trees.
How do Monarchs Find Roosts?
In the eighties some research was done to identify a marking chemical
that attracted butterflies to the Mexican overwintering roosts.
It was reasoned that if each monarch left a trace of marking chemical,
the more monarchs there were, the easier it would be to detect
the marking chemical--and find the overwintering sites. The results
were negative. By default, it appears that monarchs find their
transient roosts visually. They look for silhouettes of trees
from a feeding position, or perhaps, they can spot a riparian
valley from the air. Butterflies observed descending into a wooded
valley in central Texas suggest that they can do this. Once they
have descended, they begin their search for each other.