The other day I joined the Cabrillo College Natural History Club (NHC) on a natural journal walk through Natural Bridges State Park and Antonelli Pond here in Santa Cruz. The NHC is a student club at the college where I teach, and I attended one of their meetings early in the semester. It's a very active club, and although I'm not currently one of the official faculty sponsors I hope to become one in the future. I had a prior commitment and couldn't meet them when they started their walk, but since they were traveling at what club president described as "a nature journaling pace" I was able to catch up with them.
Monarch butterflies (Danaus plexippus) overwinter at Natural Bridges. On warm sunny days they flit about, feeding and warming their bodies in the sun. When it's cold or raining they huddle together in long, drooping aggregations from the eucalyptus trees. It hasn't been cold yet this year, but in November of 2017 I went out on a chilly morning and was able to photograph monarchs clustered together.
These two photographs are the same clump. Notice that the butterflies' wings look very different when they are closed up. The insects roost with their wings held together over the back, showing the paler, dusty undersides. I think this posture minimizes risk of damage to the fragile wings as the butterflies huddle close together to retain as much warmth as possible. As the sun warms their bodies the butterflies begin opening and closing their wings to generate additional heat for their flight muscles. The brilliant orange color of the top side of the wings is the hallmark of a monarch butterfly.
The monarchs hanging out at Natural Bridges in 2018 are the great- great- grandchildren of the butterflies that were here last year. It takes four generations to complete one migration cycle. The butterflies in Santa Cruz today emerged from chrysalises up in the Pacific Northwest or on the west slope of the Rocky Mountains, and flew thousands of miles to get here. They'll be here through the winter, departing in February to search for milkweed on the western slope of the Sierra Nevada. The eggs they lay there will hatch into caterpillars and eventually metamorphose into the butterflies of Generation 1 in March and April. Generation 1 butterflies migrate further north and east, lay eggs on milkweed, and die after a post-larval life of a few weeks. Generation 2 butterflies, emerging in May and June, continue the northeast migration, lay eggs on milkweed, and die. Their offspring, the Generation 3 butterflies, emerge in July and August and disperse throughout the Pacific Northwest and eastward to the Rockies; they lay eggs on milkweed and die. Generation 4 butterflies emerge in September and October, and almost immediately begin migrating south to where their great- great- grandparents overwintered the previous year. Of the four generations, 1-3 are short-lived, lasting only a few weeks before dying. Only Generation 4 butterflies live long, and their job is to escape the winter and survive elsewhere in a milder climate.
This truly is an extraordinary migration. Given that each individual travels only part of the migration route, how do they all know where they're supposed to go? Each individual is heading for a location that hasn't been encountered for four generations. Day length cycles are probably the primary migration trigger for each generation. I imagine that since each generation is born at a different latitude from the others and at different times of the year, day length signals may be generation-specific, at least enough so to tell the butterflies where they should go.
One of the students asked a great question: Other than the fact that they make the long leg of the migration and live longer, are there any differences between Generation 4 butterflies and the others? I don't know the answer to that. I suspect that there may not be obvious morphological differences, but there certainly are physiological differences. The Generation 4 butterflies have much greater physical stamina than Generations 1-3, and have to fuel flight muscles to travel over 1000 miles. That's quite a feat for an animal that looks so delicate! Appearances can be deceiving.