Thank you Gustave for letting me use your photo.
Lynn and I had a similar experience in October 2009 but this time it was Painted Lady Vanessa cardui rather than Monarch and I will talk about this in a following blog.
In 2002 Cristina Dockx published her PhD dissertation on the migration of Monarch's through Cuba and you can find a link to this on the species page.
The Monarch has suffered huge declines across America in recent years due to multiple factors. The numbers at their over-wintering sites in Mexico have dropped from a high of 682 million in 1997 to just 42 million in 2015. So what are the reasons?
1) The greatly increased use of the herbicide glyphosate (Roundup) by farmers in N America has reduced the available foodplant Milkweed on which the caterpillars breed.
2) The use of neonicotinide insecticides (which are designed designed to indiscriminately kill all insects) on farm crops has been shown to be a cause in the decline of bees and other insects. Even ingestion of tiny amounts of contaminated nectar or pollen that are insufficient to directly kill a bee can cause it to sufficiently lose its capacity to navigate such that it cannot find its way back to the hive. And bees are much bulkier that Monarchs which also need their full faculties to be able to navigate their way to their over-wintering sites in the autumn. Research has shown that neonics also play a large part in the catastrophic decline of the Monarch.
3) Climate change/breakdown which raises the winter temperature in the mountains of Mexico where the Monarch's have traditionally overwintered in colossal numbers means that they use more of their fat reserves which are needed to fuel the first stage of their northward migration in the spring. And last but not least-
4) The large-scale and small-scale logging of the forests in Central Mexico were monitored during the period 2001-2012 and the results were absolutely shocking. The murder of two prominent Monarch conservation activists within three days in the Monarch Butterfly Biosphere Reserve in early 2020 is a stark reminder of the extent to which big business will go to line their pockets and trash the planet in the process robbing us one of the greatest spectacles on earth.