Monarch butterfly pair on Milkweed

Monarchs and Migration

Fall is an exciting time. The changing of the seasons causes many animals to head south in search of warmer weather and the food associated with it. Birds are perhaps the most famous migrants, as about half of our songbirds fly to Central or South America to feast on insects. These would be hard to find if they stayed here for our cold and snowy winters. Several other animals migrate, including some bats and insects, like the green darner dragonfly and the monarch butterfly. Migration is so widespread that one of Delaware Nature Society’s themes for our 60th Anniversary celebration is All Things Winged!

 

Monarchs

The monarch (Danaus plexippus) is one of the world’s most iconic butterflies, for several reasons. First, its large size and orange and black wings make it instantly recognizable to adults and children alike. Second, its survival is tightly linked to milkweeds (Asclepias sp.), a group of plants famous for their milky white latex. Monarch butterflies feed on many flowers, but they only lay their eggs on milkweeds. Once the caterpillars hatch, they start munching on the leaves during which they ingest cardenolides present in the latex. These toxic leaf compounds do not harm the caterpillar or the butterfly it metamorphoses into. However, they do make them distasteful to most predators.

Female Monarch laying an egg on Milkweed
Female Monarch laying an egg on common milkweed shoot
A young monarch caterpillar on milkweed
A young monarch caterpillar

Thirds, these half-gram butterflies are famous for their long-distance migration. Although it was well-known that they flew south for the winter, nobody knew where they went. The mystery was solved in the 1970s when intrepid entomologists exploring the mountains of Mexico were stunned to stumble across trees dripping with monarchs!

A tree laden with monarchs (by Hank Davis)
A tree laden with monarchs (photo by Hank Davis)

Tracking Monarch Migration

We now have a better understanding of monarch migration routes and destinations because of a tagging program organized by Monarch Watch. This organization is dedicated to monarch conservation and gives out free milkweed plants to schools and non-profits. Volunteers throughout North America capture monarchs and place a small sticker with a unique code on their wing. If anyone who later encounters a tagged butterfly alive or dead reports the code scientists can track its journey. Monarch tagging has produced thousands of data points since its inception by Dr Fred Urquhart of the University of Toronto. The early publications are charming to read (downloadable from the bottom of this page). In the 1995 volume, our former director, Mike Riska, reports on monarch outreach and milkweed habitat restoration we were doing 30 years ago!

Monarch pink flowers
A monarch with a wing tag
A tagged monarch found dead. This is a valuable data point!

We recently held a Monarch Migration Celebration at the DuPont Environmental Education Center (DEEC), our imposing site on the Wilmington riverfront that overlooks a marsh bustling with wildlife. Hundreds of people enjoyed our live animal displays, interpretive nature programs, and exploring the marsh by canoe. The highlight of the celebration was the live tagging of two adult monarchs. The incredible migration of this small insect connects Delaware with Mexico. It is 2,500 miles between Wilmington and Angangueo, one of their wintering strongholds. Angangueo holds a Festival de la Mariposa Monarca (Monarch Butterfly Festival) every February. It is remarkable that there is a celebration in Delaware followed by another in Mexico a few months later, all for the same butterfly. Maybe our tagged Delaware monarchs will show up at their celebration!

Monarch tagging
An excited crowd watches a monarch being tagged and released

 

Declining Monarch Population

Unfortunately, monarchs are steadily declining, and the migratory subspecies found throughout most of North America is now endangered. The number of monarchs recorded in Mexico last winter was the second lowest since recording began in 1995, and many Delawareans bemoaned the scarcity of monarch butterflies and caterpillars this summer. Every fall we conduct a Hawk Watch at Ashland Nature Center where we record the numbers of different hawks migrating south, and also count monarchs flying past. The number seen during first 2 weeks of September have declined from 133 in 2022 and 96 in 2023, to just 46 this year.

Monarch butterfly pair on Milkweed
Monarch butterflies copulate on common milkweed. We need to see this more often!

Monarchs face several threats on their summer and wintering grounds, as well as during their migration. The main threat is the loss of feeding and roosting habitat to development, and a lack of the milkweeds essential for their reproduction.  There are many ways to help monarchs in Delaware, the easiest of which is to plant milkweeds. There are several beautiful native species you could plant in your yard, parks or schools to create ‘monarch waystations’. Some of these are available at our Fall Native Plant Sale on Saturday, October 5, 9 am – 3 pm at Coverdale Farm Preserve in Greenville. Click here for the list of plants available!

Swamp milkweed with caterpillar
Swamp milkweed (can you see the monarch caterpillar?). Available at the Fall 2024 Native Plant Sale!

Birds are on the move too

Migration connects both people and places because the same animal may be familiar to people living thousands of miles apart. During the DEEC event we highlighted the Motus tower located in the marsh. This is part of a network of towers throughout North and South America that detect radio-tagged animals, so we can track their movement. The DEEC Motus tower has been very productive. This year it has detected 72 birds of 16 species, including swallows, shorebirds, hawks, warblers, sparrows, and thrushes. Just this fall it has detected 27 Wood Thrushes tagged during a range-wide study that we are participating in! The migration journeys of these birds can be viewed on the Motus website. Notable journeys include a Ruddy Turnstone tagged in Brazil, a Chimney Swift tagged near Quebec City, an American Kestrel tagged in Massachusetts, and an Ovenbird tagged in Jamaica. So those birds you see flying over Wilmington may soon be enjoyed by people living in Mexico, along with our monarchs!