Monarch tags are revealing the pace and timing of this butterfly’s annual migration.

Omar Franco

Mysterious monarch migrations may be triggered by the angle of the Sun

Over 20 years, citizen scientists across North America tagged more than 1 million monarch butterflies as they flitted their way southward on one of nature’s more mysterious migrations. Now, scientists analyzing data from those journeys have discovered what may trigger them: the angle of the high noon Sun—which changes over time and as one moves closer to the equator.

That “critical environmental factor” also seems to help monarchs time their daily travels and the end of their fall migration, says Steven Reppert, a neurobiologist at the University of Massachusetts Medical School in Worcester who studies monarch migrations, but was not involved with this work. As a result, adds University of California, Berkeley, evolutionary biologist Noah Whiteman, a marvel of the natural world is a little closer to being understood.”

The annual migration of monarchs (Danaus plexippus) from across the United States and eastern Canada to one small region of southwestern Mexico has long defied understanding. Ten years ago, lab and field studies showed that these butterflies have an internal clock in their antennae that helps them navigate based on the horizontal movements of the Sun. But no one knew what the trigger for their trek was—or how they paced their daily journeys.

To learn more, a nonprofit organization called Monarch Watch began to distribute pinkie nail–size adhesive tags to thousands of volunteers, who put them on monarchs flying through their area and recorded the dates and locations of each tagging. From 1998 to 2015, more than 1.38 million butterflies were tagged, says Orley Taylor, an insect ecologist at the University of Kansas in Lawrence who started the program in 1992. After the butterflies arrived at their destination in southwestern Mexico, volunteers there searched for the tags. Altogether, they gathered more than 13,000.

For each entry, Taylor and his colleagues calculated the angle of the noon Sun at the location of the tagging. Assuming the tagging occurred toward the beginning of the migration—as observers would begin their work once they spotted the first migrants—the researchers discovered that most monarchs took flight when the Sun’s angle was about 57° above the horizon at noon, no matter where they set out. There seems to be a window of opportunity for flight, when the noon Sun is between 57° and 48°, the team reports this month in Frontiers in Ecology and Evolution.

It seems, too, that monarchs continue their journey based on the Sun’s angle. By statistically analyzing where and when the tagging occurred, the team calculated that the butterflies speed up from about 17 kilometers per day farther north to about 47 kilometers per day midmigration. Then, farther south, they slowed back down to 17 kilometers a day. That pattern follows how much the angle of the Sun changes from north to south, and it matches the speeds predicted by a separate survey based on monarchs’ nightly “roosts,” where they cluster together in trees as they rest.

The new study adds “an important piece to the puzzle” of butterfly migration—and potential decline, says Anurag Agrawal, an ecologist at Cornell University. Over the 2 decades, the number of migrating monarchs has fluctuated quite a bit, raising concerns that they are disappearing. When Taylor and his colleagues examined data from tags recovered in Mexico, they concluded that monarchs that start later in the fall—when the Sun is at a lower angle—are less likely to make it. In years where there were a lot of late starters, for example, very few tags are recovered in Mexico and populations there were lower.

Nailing down this “window” for successful migrations could help conservationists assess how external factors—including climate change—affect monarchs on this perilous trip, says Andrew Davis, an animal migration ecologist at the University of Georgia in Athens who praises the “incredible amount of effort” put forth by researchers and volunteers. “It’s great to finally see this data set being used for science.”