Skip to content

Autumn is migration season in California. We all know that, in the northern hemisphere, birds fly south for the winter and return north for the summer. And indeed, this is a very good time to go bird watching along the Pacific Flyway, as migrating birds stop to rest and feed at places such as Elkhorn Slough. Here in Santa Cruz, autumn is punctuated by the return of monarch butterflies (Danaus plexippus), roosting in eucalyptus trees at Natural Bridges State Beach and Lighthouse Field.

Since 1997 the Xerces Society for Invertebrate Conservation has been tracking monarch sightings on their migrations between the western U.S. and Mexico. They conduct a volunteer butterfly count every Thanksgiving. More recently, community science data sources such as iNaturalist provide much of the information.

Xerces Society Western Monarch Thanksgiving Count. 2019. Western Monarch Thanksgiving Count Data, 1997-2018. Available at www.westernmonarchcount.org.

This morning, before it got warm, I went to Natural Bridges to see how the monarchs were doing. I wanted to photograph clumps of butterflies dripping from tree branches. It seemed, however, that there aren't as many butterflies as I remember from previous years. The clusters were not nearly as large or as dense as they should be. And the data shown in the figure below do demonstrate a precipitous decline in monarch since 2017. We're still a couple of weeks away from this year's Thanksgiving count, and there is still a chance that the butterflies might arrive in larger numbers.

Xerces Society Western Monarch Thanksgiving Count. 2019. Western Monarch Thanksgiving Count Data, 1997-2018. Available at www.westernmonarchcount.org.

Trained observers know how to estimate the number of butterflies in a cluster like this. The numbers of butterflies at various roosting sites are aggregated to assess overall population sizes.

Monarch butterflies (Danaus plexippus) at Natural Bridges
11 November 2019
© Allison J. Gong

This morning I did see one butterfly that had a tagged wing. It was wearing a green Avery round sticker, with some writing in what looks like black Sharpie. The color of the sticker was very close to the green of the surrounding foliage, so I wasn't even able to see the sticker until I downloaded the pictures from the camera.

Monarch butterflies (Danaus plexippus), including one with a green tag, at Natural Bridges
11 November 2019
© Allison J. Gong

At first I thought the tag resulted from an official scientific project or undertaking, but it turns out that anyone can tag a monarch. The tags are used to track migration of the butterflies. There doesn't seem to be a central depository of tags and their origins, so knowing the color of the tag doesn't tell me where this particular butterfly came from.

Once the sun hits the butterflies and they begin to warm up, the clusters start breaking apart. Butterflies open and close their wings, exposing the darker dorsal surfaces to the sun and warming up their flight muscles. Sometimes they dislodge one another.

On a cool morning like this, many of the butterflies that fell out of the clump couldn't fly yet, and landed on the ground. The boardwalk is perhaps not the safest place for a butterfly to wind up, but at least in a monarch sanctuary such as Natural Bridges the visitors are knowledgeable and look out for the butterflies' safety.

Monarch butterfly (Danaus plexippus) on the boardwalk at Natural Bridges
11 November 2019
© Allison J. Gong

As I wrote before, the butterflies we see at Natural Bridges this year were not born here. This means that their survival to this point has depended on healthy conditions in the Pacific Northwest and the western slopes of the Rocky Mountains, where they lived as caterpillars and emerged from their chrysalises. This also means that planting milkweed for monarch caterpillars in California won't help the butterflies that we see here, although it would help butterflies that are destined to overwinter elsewhere. What will help local butterflies--monarchs and otherwise, and all nectar-feeding insects, in fact--is planting California native plants, to provide them with the nutrition they have evolved to survive on.

1

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.

Monarch butterflies (Danaus plexippus) at Natural Bridges State Beach
18 November 2017
© Allison J. Gong
Monarch butterflies (Danaus plexippus) at Natural Bridges State Beach
18 November 2017
© Allison J. Gong

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.

Monarch butterflies (Danaus plexippus) at Natural Bridges State Beach
18 November 2017
© Allison J. Gong

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.


%d bloggers like this: