A few weeks ago I received an enquiry from a reader: did I know what was happening with the Blue Tigers at Horseshoe Bay on Magnetic Island?
At that time all I knew was second-hand or worse but soon afterwards I saw a local ABC News report about thousands of them on the site of the old Horseshoe Bay school, which I was fortunate enough to visit with family and friends at the end of May. It was a magical experience.
Visitors walk into the shade of thick coastal rainforest and clouds of blue-black butterflies rise from every shrub and tree, settling again as soon as they feel safe, when people stop moving.
Everyone in our group was absolutely stunned by the numbers (tens of thousands according to the ABC, and I’m not going to disagree). The deep shade made photography more challenging than usual so please excuse some less-than-ideal images.
The common questions were what kind/s they were (over 90% Blue Tigers; and 90% of the rest were Purple Crows, which aren’t normally common around here), where they all came from (mostly down South, apparently), and what they were doing (just saving energy by sleeping). Having done more research in the last two weeks I can now answer all of these questions more fully.
Nearly all of the butterflies we saw were Blue Tigers, Tirumala hamata. It’s the only Australian species in its genus but is closely related to the Crows, Euploea, which made up the rest of the crowd. As far as I could tell, the only Crows I saw were the Common, E. core, and the Purple, E tulliolus.
There are five species of Crow on the Australian mainland. They are all velvety black or brown with white markings and some are very difficult to tell apart. The Common Crow, Euploea core, is a familiar visitor in our gardens, having adapted to some of our exotic plants, but the others are seen less often; they all live in the Townsville region but generally stay in their natural habitat.
What are they doing?
All of these butterflies are similar in that they live and breed in coastal rainforest and vine thickets of Queensland and the NT. They respond to the extremes of our monsoonal climate by reducing their activity as much as possible for the Dry season, which is what we’re seeing here. There’s little food around for them, and none for any caterpillars, so they find quiet, shady, damp places and simply sleep: no mating, nothing. They are living on stored body fat, so their chances of survival are lower if they are disturbed too much or too often – something to bear in mind while visiting them.
The patch of bush around the old Horseshoe Bay school, closed in the 1970s is now a “Wetlands Rehabilitation Site” according to a roadside sign. Huge old melaleucas form a high shady canopy over smaller trees and shrubs, and the butterflies love it. They appear to be quite sensitive to small differences in habitat, however, since the bushland around the Lagoon – just over the road but a little more open and therefore brighter – had almost none.
There are smaller aggregations elsewhere on the Island and on the Town Common (I found one near Bald Rock years ago), and all along the coast – and even far inland, where they will find suitable spots along the gorges.
As the Dry season comes to an end, the butterflies will begin feeding, mating and dispersing. Eggs will be laid only on suitable host plants, i.e., mostly in the tropics and mostly in rainforest, but the adults can feed on any flowers so they are free to fly south if they want to.
Where do they come from?
Many of them are year-round residents but are normally spread widely across our region. Others, however, are migratory, spreading southwards and inland during the summer and retreating back to the north for the winter. We don’t have much detailed knowledge of this movement but the Blue Tigers can travel as far south as Victoria while the Crows seem to travel no further than northern NSW. Given the sheer numbers of Blue Tigers, I reckon most of the crowd at Horseshoe Bay are returning southerners.
We do know that Magnetic Island, and even this particular site, has been a favoured overwintering location for a very long time. One friend recalls seeing thousands of butterflies here fifty years ago and Peter Valentine tells us that Captain Cook “encountered vast numbers of them when sailing up the east coast of Queensland,” before he named Magnetic Island. Numbers vary considerably according to the caterpillars’ food supply but this is the largest gathering since 2015.
This twenty-year-old article from the ABC focuses on the Common Crow but includes the other species I have been talking about. It’s excellent in itself and has links to even more information.
Wikipedia’s article on butterfly migration has quite a bit about monsoonal migration of related butterflies (Crows and Tigers) in India, as well as the best known butterfly migration, the flight of Monarchs from Mexico to USA.