Looking out my front window in suburban Wangaratta this morning I noticed a white butterfly fluttering by, and a few in the background, and I thought gee, the Cabbage White butterflies are bad this year!. But then another pair flew by and another, all in the same south-westerly direction.
Something stirred in the back of my brain about butterfly migrations so I went out into my back yard to have a look. Sure enough there was a steady stream of butterflies coming from the northeast, but when one passed close by I could see the wings were edged with black, not like Cabbage Whites at all. I'm not very practiced at identifying butterflies, especially fluttering by, and wished for a butterfly net to capture one to get a good look at it.
Pulling out my Common & Waterhouse I was a bit overwhelmed by the number of white butterflies (not entirely white, but classified as whites). I couldn't see any colour on the butterflies fluttering by, but eventually looking at the distribution maps settled on the Caper White, which has minimal colour and a mainland-wide distribution and to the north. Common & Waterhouse notes that:
"Immense migratory flights of this species frequently occur in southern Queensland and New South Wales and sometimes extend into Victoria. Entomologists at the Australian Museum are collecting information about such flights, but it is still too early to reach conclusions about their function in the biology of the species. The larval food plants do not normally grow farther south than about the latitude of Newcastle in the east and Griffith in the south-west, but migratory flights often extend well into Victoria and sometimes to southern Victoria"
Weird that the migrations are away from food plants. Why??
I put a chair up against one fence and counted butterflies passing the wall of next-door's garage while timing 2 minutes on my phone. I counted 22 butterflies passing, mostly within a few metres of the ground, but some up to maybe 30 metres. Then I paced out the distance: 23m. That's 29 butterflies per metre per hour. This was at midday, but I had been watching the butterflies passing for about an hour. Guessing that the migration was 20km across, that's half a million butterflies in an hour. What? I had to recalculate. Yep. And the migration didn't stop till mid-afternoon. It's a lot of butterflies. It was a fine day, 19°C, partly cloudy, 80% humidity, and still, so it definitely wasn't that the butterflies were wind-blown. And there were none flying northeast.
Eventually I noticed that some were stopping to feed on my sage bush, which was in full flower. They must have been hungry, because they let me get close enough to photograph them with my phone. Not the best photos, but enough to confirm the identification as Caper White, Belenois java. Interestingly the photo on the Australian Museum website seems to also be on sage bush.
Geoff Park recorded a migration on 19 November 2015 in his excellent Natural Newstead blog, and Angair Factsheet 9 describes a migration on 29 November 2004. Here is a video of Caper White butterflies in Brisbane in November 2016. I only found one reference to a Caper White migration in October at Strathbogie Ranges Nature View.