When a cloud covers the sun, the wind kicks up suddenly, or some other unexplained phenomena happens, the monarchs explode into the sky for a quick flight.
It is awesome to watch, with so many flying at once that you can hear the whispered beat of their collective wings.
Monarch butterflies in flight above oyamel fir trees, El Rosario, Monarch Butterfly Biosphere Reserve, Mexico. Canon 7D Mark II. 16mm. ISO 2500. f/16 @ 1/800 second. EV +1 ⅓.