the Migration Trail Across
As the monarchs enter Mexico, two geographic features funnel the butterflies and compress their flight path, the Gulf of Mexico and Mexico's Sierra Madre Oriental mountains. The butterflies seem to follow the Sierra Madre mountain chain as follows:
Enormous 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. The butterflies follow that bend. They change direction and head south, southeast.
When 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 central Mexico's Transvolcani Belt (19N, -100W). The monarchs 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.
Looking at 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.)
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.
At last they start to hit the Transvolcanic Mountains-- Amealco, Guanajuato, Coroneo, Guanajuato, then finally Contepec, Michoacan which is the northernmost over-wintering site.
Amazingly, the over-wintering area the monarchs have to hit is very, very narrow. The area is only 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 (-99.7W). This means that, assuming that monarchs cannot 'home' and correct the consequences of a miss, those migrants flying in from the north must struke the Transvolcanic Belt somewhere within this 1.1 degree window to find the overwintering sites!