The monarch butterfly (Danaus plexipus) migration between Mexico and the U.S. and Canada has helped make it an iconic species. The southward migration to Mexico from the U.S. and Canada begins in August with movement of the northernmost edge of the population. Additional southern migrants then join the passing migration as it makes its way towards the Mexican overwintering grounds. Though past research has suggested a time-compensated sun compass as a major factor in eastern North American monarchs ability to navigate, less is known about what affects the initiation and pacing of the migration.
Results/Conclusions
Using a large, 18-year, dataset of 1.38 million butterflies tagged during the southern migration from the Monarch Watch citizen science group, we show that the timing of the of the monarch migration is consistent with cues related to the sun angle at solar noon (SASN). The data suggest a ‘migration window” with a SASN of 57-46°, before and after which tagged butterflies have a lower chance of being recovered in Mexico. Though only 13,824 (1%) of the butterflies tagged in the U.S. and Canada were then recovered in Mexico at the overwintering grounds, those that did, demonstrated a relatively narrow interval of taggings corresponding to an 51.2° ± 3.2 (s.d.) SASN for successful migration. This relationship with SASN may indicate a migration initiation and maintenance signal.