Friday, 27 November 2015

Millions of butterflies

A Caper White Belenois java butterfly passes through the garden

For the past week there have been white butterflies passing through our garden in Canberra, lots and lots and lots of them. They are on migration moving across southern New South Wales, heading in a broadly easterly direction. The species is Caper White and during the peak number day - last Tuesday, the 17th - I counted an average of seven per minute pass through a hundred metre long corridor between our garden and those of our neighbours, which are aligned north-south. They were flying through all day, so I reckon that there were 420 passing through per hour, which would make a few thousand during one day. The main front seemed to go through that day, but there are still a few stragglers about, some of which might be stopping locally, others are still moving east.

Almost all the butterflies were males, I maybe saw two or three that looked like they might have been female,
but they never stopped so I could not be certain. Where are the females?

There were reports of them from a wide area around Canberra, so I wonder how many were in the flock? I saw similar numbers flying in the same easterly direction as I was walking and driving around the area, so the density was possibly at the same scale over a much wider area. If so, there would have been 4200 per hour crossing a one kilometre line. The plains area of the Australian Capital Territory is about 25 km wide from north-south which could have seen about 100,000 butterflies move  through (I don't know if they passed over the forests and ridges, but if so that would bring the ACT width to approximately 90 km and approximately 400,000 of them).

A stop to re-fuel

So, if they were flying for about 10 hours that day at the same density (which they seemed to do near me) there would have been approximately a million passing through the ACT in that one day. Even if these figures are wrong, there were numerous butterflies on passage for a few days, so overall there would likely have been more than a million altogether, possibly many more. And the butterflies were flying across a wider area, hundreds of kilometres wide across neighbouring parts of New South Wales, so how many million were there on migration. Where did they come from and where did they go. I know many were seen down at the coast about 200 km to the east, and many must have stopped there, but why were they moving from west to east. My friend and local butterfly recorder, Suzi Bond will, I hope, come up with the answers. She dropped me an email about half an hour after I first noticed the unusual number in the garden. She had seen them too.

How many flowers were pollinated by so many butterflies?

This sighting reminded me of a similar butterfly day I had a few years ago while driving from West Wyalong to Canberra. That was through farming country, mostly wheat and canola oil. There was what seemed to be a miriad of Cabbage White Pieris rapae butterflies along the road verges. So I counted, well estimated, how many there were per kilometre of road and for how far along the road. It was a three hour drive and there were clouds of them for most of the way. There were x between the road and the field fences per one kilometre strip and I traveled y kilometres with them at similar densities (I forget the exact detail of the count, but I do remember the total as it was so mind-boggling). I saw at least 2,000,000 Cabbage White butterflies in one day. And there were many, many more farther into the fields, there were flickers of white all over the landscape, dotting the canola crop especially. Those butterflies weren't migrating though, there just seemed to have been an enormous synchronised emergence.

Two true wildlife spectacles, played out by common garden insects. Wonderful.

A male with ragged wing edges - how far had he flown?

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