Moths and a sparrow
7th October 2004
Jeremy Smith, Station Leader (on board the Aurora Australis)
Even before the icebergs appear, and despite the monotonously watery view, the Southern Ocean can still be interesting. Although the chances of meeting another ship are decidedly slim, it is still possible to get visitors - as we did this morning.
The weather was a bit odd, mild and foggy and with only a moderate wind. We were at 47°S 152°E, southwest of Tasmania 500 km from nearest land. There had been a north wind through the night, ahead of a westerly change that reached us, with rain, in early afternoon.
I was on an upper deck watching a mooring being retrieved. It was a sediment sampler that had been sitting on the sea bed 4400 metres below us until 'pinged' and instructed to let go and float to the surface. It took an hour to come up, and was now being caught with a grappling hook and drawn towards the lower trawl deck, for removal and replacement of its samples and batteries, and ultimately to be sent to the bottom again for another year.
A moth fluttered out from somewhere below me and was carried out of sight over the side. At first I assumed it was a stowaway, disturbed from an ill-chosen resting place. Then I saw another, but still thought nothing of it. I should have known better.
Eight years ago, on Macquarie Island south of New Zealand, I was involved in an investigation of two species of moth and one butterfly that arrived on a warm upper wind one night. We had weather data from our own balloon observations, and determined that for a period of less than a day a warm upper wind had swept at high velocity, bringing these migrant insects all the way from Victoria. They had come well over a thousand kilometres, sustained by air temperatures of 5-8 degrees, warmer than temperatures at the ground where we found them in moribund condition. Still, it was a fascinating example of insect migration, and was eventually published in an academic paper.
This afternoon Noel, the senior diesel mechanic travelling with me to Casey, asked if I had seen the moths. He said he had also seen a smaller species than the chocolate-brown specimens that he and I had both observed. Pennies dropped! I confirmed with the weather observers the wind pattern of the previous few hours. Sure enough, conditions were perfect for movement of flying migratory insects from Victoria, over a distance of at least 800 km. Even if they had come from Tasmania, which is less likely, they would have flown about 500 km.
I was reminded of the bush flies that settled all over our ship as we returned from Antarctica to Fremantle last November, carried at least 75 km from the Western Australian coast on a warm, dust-smelling, offshore wind.
But I missed the best visitor this morning. While albatross were wheeling down one side of the vessel, Noel saw another bird on the other side, a small brown passerine which he said looked and sounded like a sparrow. The tired creature tried but failed to settle on the ship's rail. Even had it succeeded it was as doomed as the moths that fall into the sea to be eaten by scavenging petrels, but it is still another fascinating example of long distance animal migration.