Diagnostic Test 11

Across multiple regions, the shortest days of the year coincide with the coldest average temperatures, and both precede migration.

In a laboratory where temperature and humidity are held constant, monarchs exposed to progressively shorter day lengths display sustained migratory restlessness, while those kept on long-day light cycles do not.

A related swallowtail species initiates migration after nighttime lows drop below 50 degrees for several consecutive days.

In the field, monarchs become restless immediately after an early autumn cold front passes through a region.