Friday, 3 February 2012

Butterfly migration update

Thanks to all who've given me information so far! Numbers are definitly falling now around here, but some big waves are still coming through and that big wave of movement must be headed somewhere else... If you've been following you'll notice that new points have been appearing on the map, though more would be great, so if you've been hesitant, please do let me know. You can either add points yourself here, email or SMS me (if you know my number of course) or add a comment to the blog and I'll update things. I'm just as keen on negative data as positve ones if you're in the area - I'm getting hints that there's nothing moving in southern Tarangire, for instance, but only because I've been asking folk there and they've all been silent! Similarly, I think it's negative all over Serengeti, but I'm struggling for people to tell me that despite sending lots of messages!

Vachellia and Senegallia species in massed flower near Longido, Jan 2012
Some things I've been wondering about that might help us piece things together include information on average flight speeds for butterflies of other species - around 14kmh for Monarchs, butteflies that like a little tail wind. And between 10kmh and 6kmh for a range of other species in still air (higher speeds usually by migrant species, but not specifically recorded on migration). When our African Monarchs are on the move they seem to travel at about the same speed as the whites and they use a following breeze too, so let's assume a similar flight speed. Now I first noticed movements in Arusha on Tuesday, but it could have started sooner. Most movement has been between 10am and 5pm, giving 7 hrs of movement time. And most of the butterflies haven't been stopping much to feed. So let's say each individual has been moving for 7hrs on three days, they should have covered nearly 300km in that time. My most easterly records of the movement on the map (did I say to help me fill it in?!) are from Korogwe on Wednesday, when a notiable movement was headed NW - not as many as elsewhere at that time, but I don't know what it was like there on Tuesday or Monday. So those eastern butterflies are around 300km from me, if they followed the direction of movement we've been recording. If they flew each day, they'd be passing here about now. It's tempting to think the lower numbers at the moment represent the end of a continouos movement from Korogwe to here, whilst the peak that was here should no be about 300km further along, takingus beyond Eyasi and into uncharted territoriy. Anyone know folk in Singida or out that way who might tell us what they've seen?

The other interesting observation is that the only record I have of the bulk of the butterflies stopping to feed, rather than just a few, is in the bush north of Meru, where they're apparently completely covering the Vachellia and Senegallia species that are in flower over there at the moment. Intersting observation that - I was up in the Loliondo area a couple of weeks ago and the flowering event was truly stunning - V. tortilis and S. mellifera in amazing abundance. How widespread is that? Does that have anything to do with the moment? I'm not sure, if you're in the bush tell me how many flowers you've got around you!

The other obvious things that might explain movement patterns is weather - in particular how fast the land is drying out, and what the wind is doing. Now, whilst some have simply been flying downwind and most of the butterflies I've been told about are not flying into the wind, but they're not simply flying with the wind everywhere, there is some movement to one side or another, so it does seem like they're not just passively blowing along in a random search for food - there is some direction to it as well. Check the wind charts here and compare with the interactive map if you need convincing...

So what is going on? Well, my best guess at the moment is that conditions were pretty good in the Maasai Steppe over the last few months - we had plenty of rain in November / December, so lots of food. Now it's drying out very, very fast down there, and food plants on which to lay eggs are vanishing, so a movement began. What the precice trigger was, I have no idea - I doubt there's any coordinating event, but there might have been some initial condition that suddenly hit across a very wide area - perhaps air pressure combined with drying? I don't know but it might be worth havinga  little look at some data. Then once moving they headed initially north out of the Steppe until they saw the massif of the Pare Mountains blocking the way which alsomay have served as a sort of collecting point. Then they headed along the ridge NW, and were also blown west by a prevailing north-easterly wind. As this wind turned easterly near Kili, so did the butterflies and on they went, etc. I'm sure wherever they find suitable food plants on their way they'll be laying eggs (so we'll probably have another emergence in about a month - generation time for Belenois is about 30 days), and my guess is that's they'll probably just keep moving and laying along the way until they die. Though it would be interesting if they arrived en mass somewhere as they are reported doing in parts of southern Africa. The only way to tell is if everyone sends their records to me - no rewards beyond that warm feeling associated with being part of something... I think we should be able to combine everything into a note for some journal East African or Lepidopteran focussed - I think we've already got more information on this current movement than I can find out about any previous ones in the region and please let's get more!

Thanks to everyone for contributing so far. Keep the records coming in - and if you're just on your way back from safari, tell me where you've been this last week and what you've seen!

No comments:

Post a Comment