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.
https://www.sciencemag.org/news/2019/12/mysterious-monarch-migrations-may-be-triggered-angle-sun
