Following the Mountain Chain
the butterflies enter Mexico, geographic features compress their flight-path.
This funneling is due to the Gulf of Mexico and Mexico's Sierra Madre
Oriental mountains. The butterflies seem to follow the
mountain chain. They change direction and head south/southeast.
Getting a Lift
Sierra Madre Oriental forms a great elongated massif that stands in the
path of the easterly winds that predominate at this time of year. In short,
as easterly winds blow, the butterflies get a 'lift' from these winds.
The air rises over the east-facing mountain slopes of the multiple ranges
of the Sierra Madre Oriental. When the butterflies fly through the inner
montane valleys of the Sierra Madres, this rising air makes flight much
easier. During late afternoon, after a day when no or few butterflies
have been seen, they often quite suddenly "fall out" of the
sky and begin to nectar or to form their evening roosts.
Following the Bend
numbers of monarchs travel through the mountainous areas in the states
of Coahuila and Nuevo Leon. In Coahuila, the path crosses
Ciudad Acuna, Bustamante, and Monclova. At Monterrey,
Nuevo Leon the Sierra Madres are high. The ridges are very, very sharply
defined and they curve distinctively. They change direction. It’s
just absolutely phenomenal to see from the air! The mountains change directions
from roughly an easterly direction (running from Saltillo to Monterrey),
and they bend and turn toward Linares. And the butterflies follow that
seem to follow the mountain crest/valleys especially in area between Monterrey
and Linares, Nuevo Leon. This is a major monarch highway, through all
the little towns like Galeana and Iturbide, Neuvo Leon. The monarchs continue
moving to the south-southeast, hugging the mountains to the west of Ciudad
Victoria, Tamaulipas. West of Jaumave, Tamaulipas they pass through an
amazing portal into the inner montane valleys called the Novilla Canyon.
When south of Cuidad de Maiz, San Luis Potosi there's a pass where monarchs
go through at exceedingly high rates, probably thousands per minute.
the map, what happens next is just so delightfully confusing! They've
been traveling slightly to the east when following the Sierra Madre Oriental.
But then they come to the region surrounding Jalpan,
Queretaro, that's called the Sierra Gorda and they head south. I think
this is what's most incredible--this change in direction in the mountains.
They don't seem to have any clear ranges to follow, yet they seem to make
this turn. (The Sierra Gorda region is highly dissected by rivers and
valleys are not aligned in a north/south direction like the Sierra Madres.)
A Region with Few Sightings
We just don't
know where the monarchs are when they go through the Sierra Gorda. It's
a thinly populated, mountainous area and so not many people see them.
You can see monarchs in Jalpan, Queretaro, then again at Queretaro, Queretaro
and Tequisquiapan, Queretaro.
Reaching the Transvolcanic Mountains
At last they
start to hit the Transvolcanic Mountains-- Amealco, Guanajuato, Coroneo,
Guanajuato, then finally Contepec, Michoacan which is the northernmost
the butterflies reach deep, central Mexico they get thicker and thicker
and are easier to find because they are concentrated by the Sierra Madre
Oriental along which they fly. These mountains seem to focus the migration,
and direct it towards the overwintering sites located in the Transvolcanic
Belt of central Mexico (19 N, -100 W).
Finding the Overwintering Sites
the over-wintering area the monarchs have to hit is very, very narrow.
This is an area only is 1.1 degrees wide, in longitude. The western-most
side is at “Mil Cumbres” (-100.8 W, a large wiggle in the
road between Ciudad Hidalgo and Morelia)
and the eastern-most side is the Nevado de Toluca (-99.7
that, assuming that monarchs cannot “home” and correct the
consequences of a miss, those migrants flying in from the north must strike
the Transvolcanic Belt somewhere within this 1.1 degree window to find
the overwintering sites!
Plot the Possible Pathway
Along Migration Pathway