The monarchs started a long, staggered departure in mid-February. On Cerro Pelon, part of the colony stuck with their second roosting site above El Llano until the end of the season. Other pioneers flitted about lower altitude locations along the ravine below the meadow. Every sunny day, monarchs streamed down from their perches, turning La Cañada (the ravine) into a river of butterflies. Some were seeking out fresh nectar sources and returned to their colonies later in the day. Others started their journey north, as the roosts slowly diminished in size until finally by March 20 the last of the clusters had definitively dispersed.
Most of the monarchs had left by the time the population numbers for the 2019–2020 season were released on March 13. The bar graph took a nosedive. Many regular visitors to the colonies and most sanctuary residents expressed skepticism about the accuracy of this information. Sierra Chincua in particular seemed to have had an especially robust population this season, an impression which was not reflected in the official count. Experts abroad scurried to justify the disconnect between the massive numbers of monarchs sighted in the summer breeding grounds in 2019 versus the puny quantity reported in their overwintering sites. Was this discrepancy caused by climate change, traffic mortality or some other factor we have yet to apprehend?
There is, however, a more parsimonious explanation: maybe the count is off. Unlike the western monarch migration census, where thousands of volunteers take part, citizen science is prohibited in Mexico’s Monarch Butterfly Biosphere Reserve. Even conventionally accredited scientists struggle to obtain permission to conduct research on the monarchs in Mexico. Only one organization is authorized to conduct monarch monitoring. It’s unclear how their numbers are derived. One thing is clear: the extant methodology does not take the density of the colonies into account. And if the numbers we’re seeing are just the area of the trees the monarchs cover without any consideration of the density of their clusters, then that’s very partial information indeed. The health of the monarch migration could be better — or worse — than we think.