During the past months, as the North American monarchs overwintered, I’ve been using Journey North data to identify potential monarch coastal overwintering sites in the southeastern US. Observations suggest that monarchs in this region may be able to use these alternative overwintering sites rather than traveling to Mexico.
As the long-distance monarch migration is threatened and overwintering numbers in Mexico lag belowhistorical levels, these alternative sites could provide important refuges for monarchs to pass the winter in reproductive diapause, and could serve as a contingency against disruption of the long-range migration route.
However, identifying winter monarch habitat in the eastern US is complicated by the presence of winter-breeding resident populations. Winter-breeding has been driven by the introduction of non-native tropical milkweed, and is associated with high levels of parasite infection in monarchs, so it is important to be able to distinguish between winter-breeding sites and natural overwintering sites.
Since December 2020, Journey North volunteers across 606 locations in 8 southeastern states submitted over 5000 observations of monarch adults, eggs, and caterpillars. Using their sightings, I looked for hotspots where high densities of adult monarchs were reported that were not in close proximity to eggs or larvae, and eliminated locations where eggs and larvae indicated winter-breeding.
Then, I dug into the comments that Journey North users provided with their sightings, making sure there were no sites included where volunteers described mating or egg laying behavior during the winter. Finally, I eliminated all sites with less than 5 reports and selected a minimum cut-off for the density of adult monarchs reported.
This left only eight top-ranked sites across Florida, Georgia, Alabama, and Texas. This winter, collaborators and I visited three of these sites to look for monarchs and evaluate habitat characteristics. Thanks to the keen observations of Journey North volunteers, we are learning more about these potential monarch winter refuges!
Charlotte Hovland
Altizer Lab
University of Georgia Odum School of Ecology
