Do Monarchs Migrate Across the Gulf of Mexico?
Contributed by Dr. Bill Calvert

The evidence and lack of evidence for monarchs crossing the Gulf of Mexico is considered below. There have been no tag recoveries that clearly indicate that they do cross the Gulf. Please read it and make of your own mind about whether or not they do. I think you will find that weighing the evidence is what makes the process of science so exciting.

Also see thoughts from Mr. Don Davis, an authority on monarch butterfly tagging. >>

Evidence for monarch crossing the Gulf of Mexico:

1) Monarchs travel far and wide. Recoveries have been made in Cuba, the Bahamas and the Dominican Republic. They have been observed in England apparently having crossed the Atlantic. Crossing the Gulf of Mexico (a flight of as much as 900 miles of open water) miles may not be much of a task for them. If they thermaled up to 3000 feet and caught a strong tail wind of say 70 miles per hour and flew for 10- 12 hours, they might just make it. No one has ever recorded a monarch traveling so far in one flight period, but it is at least theoretically possible!

2) Monarchs are practically unknown in the Yucatan Peninsula until sometime around mid-October. After mid October, monarchs are said to be abundant. (The Danaid population up until that time consists of Queen butterflies, Danaus gilippus.)

3) Ships and off-shore drilling platforms periodically report monarchs thermaling above smoke stacks or landing on drilling platforms in many places in the Gulf of Mexico. These butterflies may be in transit across the Gulf.

Evidence against monarchs crossing the Gulf of Mexico:

1) Flying over the Gulf of Mexico is simply too far for an insect to manage. The sightings in the Gulf are probably lost butterflies that have been blown off course. The probability of their connecting with absolutely correct winds to make the passage in one flight period is very small.

2) It's true that monarchs are far ranging fliers, but they are easily blown off course and arrive in some strange places. In other words, the recoveries in the Caribbean and their presence on offshore platforms may represent an unimportant (small) part of the population that has been blown off course.

3) Florida has a number of winter aggregations that may represent migratory monarchs that have strayed unto the Florida peninsula or perhaps have been blown there from parts of the Florida Panhandle. Perhaps the most famous of these is at Honey Moon Island north of Dunedan Florida on the Gulf Coast. These aggregations arrive in September/ October and leave in January/ February. The females leave first. In this way arriving earlier and leaving earlier, and the females leaving first - they resemble California overwintering monarchs more than the ones who overwinter in Mexico.

4) As dramatic as the sighting of monarchs apparently beginning a long trek across the Gulf seem, as many sightings have been made of monarchs apparently struggling to regain the shore. It may be that monarchs are blown off course (out to seas) and then they must struggle hard to come
back to land.

There are so many sightings of monarchs in unusual places that we must take them into consideration when studying the migration. But perhaps the first question we should ask is, are these apparent anomalies important? That is, do they represent an important digression from the main pattern of migration, or are they so few in number, that they are of no real consequence?

If the latter, what functions might the "anomalies" serve? We all know how important population variation is to natural selection. We also know that more variants are not viable. Perhaps the monarchs that are blown off course become founders of populations able to exploit unknown breeding areas heretofore unavailable to the main population.

You have read the positive and negative arguments about anomalies of the migration. The evidence is before you. You must use it to make up your own minds about the "truth" of the situation. This is how science works. It is seldom as certain as it is made out to be. This is what makes it so exciting to scientists. The burden of determining what to believe is on you.

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