Migrating Monarchs

Being self isolated has added to my knowledge of natural history - little did I realise how few species populate the garden and how slowly the populations change, or how many specimens fly in then immediately depart. Photographic opportunities are fleeting.

The  isolation has at least one upside, I can now catch up on processing a large backlog of photographs. Not least my trip to Cape May in New Jersey back in late September, which was primarily to witness the Eastern Flyway, but afforded me the opportunity to witness another key migration, that of the Monarch Butterfly, Danaus plexippus.

There are few true migrating butterflies, the two that immediately come to mind are Painted Ladies and the Monarch. Both occur in spectacular numbers and travel huge distances. In the Monarch's case millions migrate each year, all heading to one location - the forest mountains of central Mexico. A journey entailing distances of over 2000 miles. Of course the attrition rate is high, unable to tolerate prolonged freezes, many perish. However, the cool mountain temperatures in Mexico allow the butterfly to spend the North American winter in hibernation. In early spring rising temperatures break their sleep and they start the northerly migration, As the Milkweeds of Northern Mexico and the southern United states appear, the larval host plant, the females lay their eggs. It is the progeny of this first wave that will repopulate North America. Two or three non migratory generations occur - then, in the Autumn the migration begins again and this is what I was pleased to observe at Cape May.

Researchers at Cape May are monitoring numbers to learn how varying environmental factors are influencing the migration. A standardised census count is made three time a day between September 1st and October 31st. Thousands of Monarch butterflies are tagged each year in Cape May. The tags consist of small pieces of coded adhesive paper place on the wing which does not change the way the butterfly behaves or flies. Monarchs are not slow during the migration, one butterfly tagged in Cape May was monitored in Georgia three days later - a distance of 558 miles.

Come the evening they roost in huge numbers.

Reproductive hormones are turned off for the duration on the migration - these are just good friends.

"My Monarch"

I had the opportunity to have a lengthy chat with one of the researchers and my burning question was about the genetic make up of the population. Occasionally Monarchs turn up in UK, where they come from is anyone's guess, whether arrival from west or east or the abhorrent "confetti" - who knows. However, I had imagined that these butterflies and those sedentary populations on the Atlantic Islands would have significantly different ancestry. The answer - they are all genetically identical - the mind boggles.


Many thanks to the staff at Cape May for the information on the Monarch and their patience with an inquisitive visitor.