Time to Revisit Views About Monarch Migration?
Unusual Findings During the Winter of 2005-2006

Contributed by Andy Davis and Sonia Altizer
The large numbers of monarchs seen during the winter months along the Gulf and Atlantic coast is indeed interesting. We are not aware of previous studies documenting this high number of sightings, and these reports may stimulate scientists to revisit common views of monarch migration and overwintering behavior.

First, however, we must always consider how Journey North data is collected. To what extent are we seeing changes in reporting frequency by the observers, and to what extent are we actually seeing changes in monarch behavior?

Even with "effort" in mind, however, it still seems very likely that two key factors could have affected the numbers of monarch winter sightings. These observations point to human activities that may be causing changes in monarch behavior:

1) One relates to climate. Given the milder winter climates we’ve had recently, monarchs could survive longer and even reproduce in areas along the coast. Even though monarchs cannot withstand prolonged freezing temperatures, they appear to survive occasional bouts of freezing or near-freezing temperatures for short periods. Therefore, it’s possible that some migrants flew to coastal sites in the fall and remained for the winter for an extended stay.

2) A second factor relates to the planting of tropical milkweeds, especially bloodflower (Asclepias curassavica). Although almost all native milkweeds in the eastern U.S. are perennials that die back in the fall, tropical milkweeds can persist in gardens up through a hard frost. The combination of milder climates and the presence of tropical milkweeds could provide conditions suitable for monarch breeding during the winter months, as occurs in South Florida and much of the new world tropics – and as has been reported in parts of TX during the winter months.

Time to Reconsider Traditional Views?
Readers might be familiar with the traditional view that fall migrating monarchs are in a state of reproductive diapause and do not mate or lay eggs until the spring. However, it’s possible not all monarchs respond in the same way to the same cues, especially given the wide geographic distribution of monarchs, and the variety of climates to which they have acclimated. That’s to say that if the conditions were right last fall, and if there was milkweed available in people’s gardens, it is possible that some monarchs could have stopped migrating and instead remained for the winter, in some cases breeding.

How Did the Monarchs Get There?
In terms of how the monarchs arrived along the southeastern coastline to begin with, there has been some debate among scientists as to whether the monarchs seen in the fall along the East Coast are actually following the coastline to Mexico, or are blown eastward while tying to head south along an inland route. Some debate concerns the probability that coastally-migrating monarchs ultimately reach Mexico, and some research showed that monarchs tagged on the coast have a smaller chance of being recovered in Mexico than those tagged farther inland. If some monarchs do indeed follow the coastline they would inevitably end up in coastal areas of the southeast.

Disease Concerns
Although Journey North participants may be excited to have monarchs stay all winter-long as garden visitors in coastal sites, it is not clear whether this is a positive sign for the eastern population. One possible outcome is that extended breeding during the fall and winter could lead to the accumulation of parasites in the monarchs’ environments, leading to higher infection rates and greater parasite spread to returning eastern migrants from Mexico.

Perhaps Journey North participants can contribute more pieces to this puzzle of coastal winter sightings of monarchs, and how this relates to planting of tropical milkweeds, climate variation, or changes in reporting frequency.

Education Standards >>

University of Georgia Scientists Catching Monarchs
Andy Davis and Sonia Altizer
Location of Monarchs
Winter 2005/2006
A Warm Winter
January 2006 much warmer than normal >>
Related Stories
Spring 2006
Wintering in Virginia
Tagged in September, 2005 and recaptured in March, 2006! >>
A Subtle Surprise: Look at the East Coast! >> 

How could assumptions interfere with good science? >>

The Gulf Stream
keeps temperatures warmer along the coast to North Carolina
Related Story
Spring, 2002
Early sighting in North Carolina March, 2002 >> 

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